Roy Hattersley boasts the type of benevolent salmon-pink face that would look good framed by a uniform. Perhaps the dark blue and maroon braided uniform of the Salvation Army? Major Hattersley, the evangelist regaling the faithful. Major Hattersley spitting pellets of zealous emotion, espousing the need to go out among the poor, the degraded, the lost. Tambourines, hymns, ribbons, ladies in bonnets crying ''Onward Christian Soldiers!''.

This religious fervour of a vision has been provoked by Mr Hattersley's latest biography, Blood & Fire. The biography focuses on the union of the Salvation Army's founders and great social reformers William and Catherine Booth. Two underrated Victorians whose active evangelicalism and commitment to the poor drew such violent opposition from the establishment, the couple have been largely footnoted in history despite their universal legacy.

Sadly, the Major Hattersley vision is dispelled by the sight of a plump man in sensible corduroy trousers and v-neck jersey strolling across the lobby of Edinburgh's Balmoral Hotel. Roy Hattersley, it emerges, is more disturbed by the ubiquity of Carol Vorderman on television than entertaining my vision of Major Hattersley banging a tambourine. ''Since I am an atheist, that is a highly unlikely scenario,'' says the journalist, author, and politician, good-naturedly. ''I would love to be a believer but, No, this project has not changed my views - one iota.''

Despite immersion in the lives of nineteenth-century England's fastest-growing religious movement, the pragmatic Yorkshireman remains impervious to any queries of a blinding road-to-Damascus conversion while researching his book. This, despite a leading Salvationist placing his hand on Mr Hattersley's head at a rally and declaring that ''there are many people who are Salvationists without realising it themselves''. He elicited a wry smile from the amused, white-haired atheist seated beneath him. He concedes, though: ''You understand how the Victorian poor were carried along by this because I did feel the moral pressure to respond. If I had walked to the front of 4000 people and said 'brothers and sisters - I believe' I would have sent them all home happy.''

As a former Deputy Leader of the Labour Party, Mr Hattersley is well versed in resisting the sway of the majority. But, if not a convert Salvationist then, where were the seeds of the Booths's biography sown? Amid the unlikely surroundings of the South Yorkshire Engineering Employers' Federation annual dinner. ''I was seated next to the federation's chairman who was not drinking - very unusual,'' he recalls amused. ''It transpired that this man was a big noise in the Salvation Army as a layman. He had read my novels, knew I was interested in nineteenth-century England, and wanted to invite me to write something on the Army since they were in the middle of re-organising their heritage centre.''

Initially, the fire of inspiration was a damp squib. But after consulting his publisher, who told him there had not been a biography of William Booth for 80 years, making tentative research efforts and realising that the evangelical promised more thrills than the contracted book on New Labour, Hattersley took the plunge. ''Initially, I was going to concentrate on William Booth and the Victorian poor. He wrote the pamphlet In Darkest England and the Way Out, which was the first plan to tackle the industrial poor,'' and, he adds cheekily, ''quite an anti-New Labour book!'' However, it was the discovery of Catherine Booth that sealed Hattersley's resolve. ''I was absolutely taken by her,'' he smiles. ''I started off thinking she was one of most extraordinary women of the nineteenth century and ended up thinking she was 'the' most extraordinary woman of the nineteenth century.''

Blood & Fire would be a slight book without the inclusion of Catherine Booth's life. Whereas her husband, William, emerges an autocratic father of eight children, many of whom he alienated with his humourless, harsh, and autocratic manner, Catherine is portrayed by Hattersley as the intellectual powerhouse of the two, the strong-willed and progressive Victorian whose forthright views on prostitution and women's equality were germinating a century before their time.

Hattersley is on a roll now. His devotion to Catherine is obvious in the unstoppable litany of her admirable characteristics. ''She was extraordinary in that she had to maintain the life of a proper Victorian wife - with eight children in 10 years - she sacrificed, subliminated, and submerged herself. At the same time this totally self-educated woman was an extraordinary authority on biblical texts, she argued for women and men to be equal in the sight of God, she argued for women's right to preach, to have authority over men, and for women to be ordained in the Salvation Army. Socially, she believed the men who exploited prostitutes should face assault charges, not the women themselves. Where William was anxious about antagonising the rich and famous, she took them on in their own homes,'' he wonders. ''I can't think of a Victorian woman who did anything remotely like that.'' He confesses,

with a twinkle in his pale blue eyes: ''I would run away with Catherine but I'd run away from William!''

Superficially, two individuals at opposite ends of the personality pole. What drew them together? For Hattersley, who trawled through the couple's thrice-daily letters to each other in the British Library, he believes ''their mutual belief in the duty to redeem and immense piety'' proved a solid foundation on which to build both a marriage and a religious movement.

No doubt, their marriage was strengthened in adversity. The Salvation Army's outspoken stance in appealing to the working classes, did nothing to enhance the Booths's standing within the establishment of the time. Cold stares and verbal pillorying at the hands of the press, magistrates, and the police were not the only hurdles the Salvation Army had to jump through for the first 30 years of its existence. The ''soldiers'' were physically persecuted by thugs hired by brewers who took exception to the Army's preachings on teetotalism.

This is only part of the reason their biographer believes they have been so underestimated as eminent Victorians. ''There were many reasons why the Booths were disliked,'' he begins. ''First, the establishment feared a revolution in this mass organisation led by a self-proclaimed general. Secondly, William was so obtrusive. The establishment was embarrassed by an army shouting 'God Loves You' and 'Redeem Yourself Before It's Too Late'. Thirdly, the Army were thought rather ridiculous in their uniforms, bonnets, and military titles of General Booth.''

For Hattersley, image and the sway of a one-sided establishment could not obscure ''the appeal of Old Labour values like getting rid of poverty and social reform'' in the Booths's path to evangelical righteousness via their own blend of Marxism and Methodism. Despite his avowed atheist status, Hattersley reveals his next project is on Wesleyan Methodism. Alas, the book on New Labour looks like it's been shelved again. Roy Hattersley doesn't seem too perturbed by the prospect.

n Blood & Fire - William and

Catherine Booth and Their Salvation Army by Roy Hattersley is published by Little, Brown at #20.