If we ever have a fascist coup in Britain, it won't be run by poor white men trying to improve their sad lives by threatening people with dark skins, but by upper-class Christian gentlemen who talk as though they come straight out of a John Buchan novel. Peter Day's account of the rise of Franco is full of them. Without them, Spain might have remained a democracy after 1936.

Hugh Pollard, who hired the plane that took Franco from the Canaries to north Africa, was typical: an upper-class Roman Catholic, he knew he had the support of the princes of his church, for the Archbishop of Westminster, Cardinal Hinsley  the leader of Britain's Catholics  said that the Spanish civil war was "in essence a contest between Christ and Antichrist".

Those who know something of British fascism in the 30s will recognise some of Pollard's friends and co-conspirators. Major General JFC "Boney" Fuller makes a cameo appearance in Day's book, meeting Franco and greasing the wheels of business with him. This is the same "Boney" Fuller who attached himself to Oswald Mosley, eventually leaving the British Union of Fascists, not from disagreement with its views, but from disillusion with Mosley's leadership qualities.

We also meet briefly, but inevitably, with Captain Archibald Maule Ramsay MP, in the Carlton Club in June 1939, offering MI6 chiefs what Day calls "a tour d'horizon of the gigantic conspiracy being engineered against Gentiles worldwide". Ramsay, a stalwart of the Right Club, eventually went a little too far, and spent the war years in prison under the wartime Regulation 18B which allowed the government to imprison those who it thought might endanger national security.

The difference between the likes of Ramsay and Fuller on one side, and Pollard and his shady associates from the top echelons of British society on the other, was not one of high principle. If Pollard and his friends never actually signed up with Mosley, it was not because they had any fundamental disagreement with him, but because they were more realistic about Mosley's chances of achieving anything.

Day traces the British establishment's aid to Franco, starting with the chartering of a plane that enabled the general to be in the right place to lead a revolt against the government. Once the civil war was under way, Britain adopted a stance which the Labour peer Lord Strabolgi called "malevolent neutrality". A stream of pro-Franco lobbyists were warmly greeted at the Foreign Office  many of them British businessmen poised to make a killing out of a Franco government. While hundreds of young men, and some young women, went secretly to Spain to fight for the republicans, a much smaller number of much more well-heeled people went to fight for Franco. Franco also had the ingenious idea of charging £8 a head to take foreigners to scenes of nationalist triumph, and hear stories of republican atrocities. One frequent visitor was Arnold Lunn, scion of the family that founded and owned the Lunn Poly travel agency, which in the 1950s pioneered package holidays to the Costa del Sol and Majorca.

But it was not until the second world war that Franco really started coining money out of his British friends. The British could have tried to profit from the fact that Franco's government was full of jealousies and faction-fighting. Instead, Churchill's policy was to keep Spain out of the war by lavish bribes to key people in Franco's government, including Franco's brother-in-law and perhaps Franco himself. A huge slush fund was administered directly by the British embassy under Sir Samuel Hoare, and the regime, which was corrupt as well as brutal, made itself rich and unassailable at the expense of the British taxpayer.

Day's prose sometimes echoes that of the writers from whom his characters seem to be drawn. The book opens with a resounding sub-Buchan sentence: "The sharp tang of orange cut through the whisky fumes in the cramped, stifling cabin of the Dragon Rapide." Day continues in this vein for quite a few pages  Pollard's daughter, "with her foxy good looks and fearless riding to hounds," had "quickly grown to admire the sandy haired, freckled young navigator". Fortunately, within a few pages the author buckles down to the serious business of telling an important story. He has put information that is already in the public domain together with the fruits of his own research to produce a short, compelling book which demonstrates clearly that, like many other brutal dictators all over the world, Franco partly owed his long, dark reign to powerful friends in Britain.