In loving memory of a man called Hoss: Fans of old episodes of Bonanza are made to feel like one of the family when they join the Fifth Cartwrights. Rose Rouse reports

Alan Grant rides a 1955 BSA Bantam 175cc as a motorbike courier in London. It ain't the Nevada outback but, hell, it'll do. 'Being a courier is the nearest thing to being an urban cowboy,' says the founding member of the Fifth Cartwrights (or, to us lay people, the Bonanza Fan Club).

Grant, 27, spent Sunday afternoons in the late Seventies curled up in front of the television, watching wholesome Ben Cartwright and his three sons (all, scandalously, by different mothers) defend their ranch, the Ponderosa, against the generally bearded forces of evil. 'Bonanaza was a good church substitute for lost souls like me,' he explains. 'There are four Cartwrights, so the viewer is effectively the fifth family member. Our members are often only children or from divorced parents.'

The release of the Bonanza video tapes in March - Bonanza was a number one show on American television in the Sixties - has caused a surge of interest in the Fifth Cartwrights. At the last gathering - at Fungus Mungus, a vegetarian restaurant in Battersea - 300 people turned up. 'There were stetsons everywhere,' recalls Grant, 'someone came on a horse from Balham and there were gun fights with water pistols.' ('We are sad cases,' he adds.)

Grant's interest in Pa, Adam, Hoss and Little Joe is mainly 'philosophical'. His preoccupations centre on the moral dilemmas presented by the Cartwrights' struggles to do their best. Upright citizens who avoided liquor, never started fights and had only the best of intentions towards women, the Cartwrights nevertheless found themselves at the centre of plots knee-deep in issues like racism and sexual violence. 'I like the programmes because they never came up with black and white answers. The writers were dealing with questions at the heart of American society, but they never went for the cheap jugular like Wagon Train.'

Members - 700 of them to date - like to come up with alternative endings to programmes, question the decisions of patriarch Pa and discuss progress in the Hunt for Hoss's Hat. Fifth Cartwrights are desperate to locate Hoss's legendary headgear, a quest that has already taken Grant to Spain. 'There are some old studios which were used for filming outdoor location shots near Madrid,' says Grant. 'I was told it was in their prop room.' It wasn't. Two computer buffs have spent years working out the exact measurements of Hoss's head from a computer-enhanced image, so if they find the hat, they will at least know whether it is his or not.

Meetings are irregular, and only called at the last minute. But the opening ritual is always the same. Specially drawn maps of the Ponderosa are burned to invoke the beginning of the programme, where a similiar map goes up in flames. Some of the members get dressed up. 'Sometimes there will be arguments over the definitive costume for each character. For instance, Little Joe is often seen wearing a rakish scarf, while mysterious Adam frequently wears black leather.'

Although reluctant to don cowboy gear himself, Grant can be heard plucking out the Bonanza theme tune on his guitar. 'It was only performed once by the cast,' he says 'and that was in the first episode. Apparently, they couldn't sing so it was changed to an instrumental and the original has been lost. We are looking for a copy of it.' Frank Zappa, another Bonanza fan, used to strum the theme tune during his live performances.

The Fifth Cartwrights will be at Glastonbury Festival in June, serving baked beans, playing C&W music and setting up a barber's chair. They will not be as out of place as you might think. 'There was a legendary drugs scene where Hoss eats some hallucinogenic mushrooms and sees somersaulting leprauchauns,' says Grant. 'He doesn't do it again, but he doesn't denounce it as evil either. It was presented as an experience.'

The Fifth Cartwrights can be contacted at Fungus Mungus, 264 Battersea Park Rd, London SW11 3BP.