While most people know that Barbara Vine is the pseudonym of Ruth Rendell, the much-garlanded "queen of crime" and Labour peer, many do not understand how Rendell's Vine novels differ from those written under her own name. They are often characterised as darker, more psychologically complex and so on, but it is easier to think of them as murder mysteries in reverse: everything seems clear at the start, but mysteries emerge and tensions rise until the revelation of a violent or shocking act, often committed far in the past and hidden from the light. There is no detective, and often no real perpetrator; Vine is so good at sweeping us into the minds of ordinary people that she can make us react with horror to acts that, out of context, might not seem all that bad.

The Birthday Present is full of context - perhaps too full. It follows Ivor Tesham, a breezy but ambitious young Tory politician in the last days of Margaret Thatcher's government. As he confesses to his brother-in-law, Rob, one of the book's two narrators, he is having an affair with a married woman, Hebe. You can glean all you need to know about Ivor's character from the fact that he proudly tells Rob he is thinking of asking Hebe to leave her husband and two-year-old son - not to move in with him, of course, but to set her up in a flat in Mayfair. Rob, who is devoted to his own family, says that Hebe would at least want to bring her toddler along. "That would be a bit of a drawback," says Ivor.

So when Ivor conceives a plan to pay two minicab drivers £500 to stage a birthday "adventure" for Hebe, involving handcuffs, a blindfold and a mock kidnapping, you can't help but feel it is bound to go wrong. As Vine points out several times, this was the era of Tory sleaze and political arrogance, when a Labour victory seemed a laughable prospect.

Indeed, the time and place are so much stressed that the story seems secondary. It is not giving too much away to reveal that the car used in the kidnap collides with a lorry, leaving Hebe and one man dead and the driver comatose in hospital. A gun is found in the wreckage - a real one. Ivor, of course, is terrified that his role may leak to the press. The question of what steps he will take to stop this happening - notably in the case of the injured survivor, who may wake up at any moment - drives his half of the book. How far, in private, is this respected public figure prepared to go?

There is another major player in the drama: Jane, the friend Hebe used to say she was meeting when she was seeing Ivor. Jane's diary makes up the other half of the book, and it is by far the more interesting. She exemplifies that type which Vine, and her counterpart, PD James, do so well: the solitary adult, lonely for too long - bitter, emotionally stunted, swinging between self-deception and self-pity. Jane is spiteful and callous yet pathetic, and we feel for her. There is a marvellously evocative description of her bedsit, with its "minute fridge, baby oven and mini electric kettle". Her involvement as the "alibi lady" adds a further touch of suspense, since she is unbalanced from the start and seems to be getting worse.

The sad truth, however, is that all these promising sparks do not result in fireworks. There are plenty of dark hints and false leads, but the twist in the tale leaves most of the loose ends hanging. What's more, the suspense worked up so potently in earlier Vine novels is not in evidence.

Perhaps that is because her favourite theme, obsession, never comes into play. Ivor is ruthless but pragmatic and there is not much that is ominous about that, even in a politician. Or is there? Rendell spends a lot of time in the House of Lords - perhaps she knows something we don't.