The balti was invented in Birmingham by Pakistani and Kashmiri restaurateurs to appeal to the tastebuds of their white customers, but it is now an authentic dish in its own right. So too, argues Ziauddin Sardar in this thoughtful study, British Asians have had their distinct identities formed in Britain. Unpacking the "British Asian experience" is a mind-boggling business, and Sardar does justice to its complexities, albeit with a post-7/7, British Muslim emphasis. Asia is a vast continent ("The generic Asian exists only in the mind of Anglo-Saxon folk," observes one wry academic), so the term "British Asian" only serves to obscure multiple ethnicities. The quest at the heart of this book is to find identity within difference. While some on the right call for forced integration, Sardar more elegantly slips into the role of disciple at the feet of Lord Bhikhu Parekh, whose redefinition of multiculturalism makes one wish everyone possessed his calm logicality. In sum, there is no static, fixed Britain, but a dynamic, never-ending process of becoming-Britain.

Nick Griffin once told me he likes a curry. It was meant to be dismissive, a way of reducing the Asian contribution to British life to a single dish; the ultimate in back-handed compliments. Ziauddin Sardar likes a curry, too. But for him they're just the start of a journey. It's well known that balti, like most curries served in restaurants, is an invented dish. Balti means bucket, and the Indian version is used to carry water, not cook food. But Sardar finds in this invention a reason to celebrate; balti suggests that "the British Asian community has the ability to reinvent itself, repackage tradition and reposition itself in relation to British society". It has also played a part in reshaping its host culture. In Birmingham, he observes white diners enjoying spicy baltis in a flock-wallpapered "traditional" curry house, while Asian Brummies opt for blander curries in a "modern" restaurant with black leather chairs and brilliant white walls - both taking the reinvention they desire from the dish.

Balti Britain tells the history of Asians in the UK, from the 17th century to 7/7 and the terrorism arrests of the last few years, mingled with a history of his family's arrival in Britain. Sardar's recurring theme is that a nation that doesn't explain its imperial past is haunted by it. History lessons that go straight from the Tudors to Hitler miss out the crucial centuries in which India built Britain's wealth, shaped its culture and fought for its survival in two world wars. The result is a nation so amnesiac that it treats Asian immigrants - who thought, like my father, that they were coming to empire's mother country - as strangers. Sardar points out that successive generations have been treated as strangers in their own home, despite being born here.

In a brilliantly reported passage on the Lancashire mill town riots of 2001, Sardar takes the broad history - a cotton trade built on the enforced ruin of India's mills and a migrant community then left stranded when Britain's spinning industry declined - and combines it with detailed observation of two neglected cultures pitted against each other. From a memorably described scene of being the only Asian in an Oldham pub, he travels to the Pakistani enclave, where scuffles spill over into a riot: "I could see the mob attacking anyone Asian. They were banging on doors, throwing bricks through windows, and urging the 'Paki bastards' to come out." It's fashionable to criticise multiculturalism, but as Sardar points out, the one thing the Asians in Oldham didn't lack was British identity. There was no failure of multiculturalism in the mill towns because it didn't exist - there was only biculturalism; white and brown Lancastrians with identical accents locked in combat over scarce resources.

Exploring his own history yields an unwelcome surprise: his paternal grandfather Ahmad Ullah Khan was one of the British empire's enforcers. Khan's first military action was in the second Afghan war, "meting out punishment on behalf of Victoria Imperatrix". It also emerges that an aristocratic British woman who regularly visited the family when they first arrived in Britain was the Dowager Lady Birdwood, a fascist sympathiser related by marriage to the field marshal who commanded his grandfather. Sardar's discoveries lead him to appreciate the extent to which the empire reshaped every country that became entangled in it. "The question that emerged was not how it was possible to be Ahmad Ullah Khan, servant of the Raj. The real enigma was how could I, Ziauddin Sardar, ever have been considered 'new' to Britain?"

In Balti Britain, Ziauddin Sardar has set himself a daunting challenge: to capture what he describes as the 'vibrancy and mind-boggling diversity of the British Asian experience'. There are arguably few writers better qualified to meet this challenge than Sardar, a distinguished author, broadcaster and commentator for whom the British Asian story contains his personal history. He is, he writes, 'a fully participant observer ... an agent in that which I seek to describe. My report could not be an objective exercise; it had to encompass a personal reflection on all that I am, and how I came to be here and now in Britain.'

Sardar not only travels across the country from Leicester to Bradford to Glasgow, but also digs deep into his own life. He begins in Birmingham, Britain's second city and home to more than 50 balti restaurants. Like curry - and indeed the word 'Asian' - balti (which means 'bucket' in Urdu) is a generic term, in this case one that was basically cooked up by enterprising restaurateurs to fool whites into believing they were eating something significantly different from regular curry. 'By attaching a different label to what is basically the same food,' Sardar writes, 'the Indian restaurant reframed its image', demonstrating that 'British Asians can be authentic to themselves, reclaim their history, in a number of different and innovative ways'. Thus for Sardar the development of balti cooking serves as a metaphor for the way the relationship between white and Asian Britons has developed.

The balti metaphor also rather neatly describes Sardar's book, for this is something of a bucket history. Sardar throws in big chunks of reportage in which he delves into arranged marriages and Muslim radicalisation. There are slabs of memoir as he recalls arriving in London as a boy in the cold dark winter of 1960, growing up in Hackney and experiencing racism and later the joys of becoming a husband and father. And finally he sprinkles the mix with generous lashings of polemic. It should make for an enthralling read, but as a study of British Asians, Balti Britain is only fitfully successful.

One problem is that Sardar tells us he wants to celebrate the vibrancy and diversity of the British Asian experience, but most of his actual encounters are with academics, professionals and other writers; there is a surprising lack of street-level reporting. The book also claims to be a 'journey through the British Asian experience', but this is largely a journey through the British Muslim experience. In the aftermath of 9/11 and 7/7, this bias may be inevitable, but it undermines Sardar's stated ambitions. He ignores many truly fascinating aspects of modern British Asian life - mixed-race relationships and their impact on identity; homosexuality; the dilemmas of how to deal with ageing parents; the impact of Asians not knowing how to speak their mother tongue - and instead revisits subjects such as the rise of Indian restaurants, the success and impact of Goodness Gracious Me and East is East, the dilemmas around arranged marriages and the rise of religious identity. All these topics have been thoroughly explored before, and nothing Sardar says feels especially insightful or original.

On arranged marriages, for example, he approvingly quotes a young professional woman in Bradford who says she supports arranged marriages because: 'I don't want to humiliate myself by dancing to the tune of the dating game.' Sardar adds that 'however much women are supposed to be free, everywhere they are in chains to the same underlying message: dress, dye your hair, make up your face, buy the right perfume, and most of all be shapely, diet yourself to misery or starve yourself to death in a land of abundance and plenty, and all because this is necessary to get a man'. Here Sardar the polemicist is, one suspects, drowning out Sardar the journalist. That woman in Bradford may not have wanted to 'dance to the tune of the dating game', but anyone who has visited hugely popular matrimonial websites such as Shaadi.com will know that superficiality and vanity are not exactly unknown in the Asian community.

Balti Britain also suffers from a flat writing style; one favoured technique is to reproduce lengthy question-and-answer sessions between the author and an interviewee, which Sardar tries to spice up with the liberal use of adverbs. When his son asks if it's acceptable to support Pakistan, his father replies that it is because 'you are balancing your patriotism with the weight of history. Cricket was the creation of the age of Empire. It was exported to the colonies as a way of civilising the natives.' This rather stilted exchange fails to address the more intriguing question: if his son could play for a national team, which country would he want to play for?

British Asians are used to wrestling with questions of identity, so it is perhaps fitting that Balti Britain suffers from its own identity crisis. Although his book aspires to be panoramic and polemical, Ziauddin Sardar is best when he is being personal and passionate. His generation, he admits, were uneasy in seeing themselves as British but their children 'are as naturally British as eating balti'. It is a surprisingly hopeful conclusion to an ambitious and provocative book that deserves to be read as the first draft of the history of Asians in Britain today.

· Sarfraz Manzoor's memoir, Greetings from Bury Park, is published by Bloomsbury