Somehow the space adventure series `Blake's Seven' never quite had the optimism and derring-do of `Star Trek'. The space warriors were altogether more disillusioned and despondent. Still, writes Robert Hanks, it's become a minor cult, a status which will be further enhanced in a one-off radio drama this Saturday.

If you wanted to sum up the relative position of Britain and America in this century - the ebbing away of the pink areas of the map, the fading of national self-confidence as Uncle Sam proceeded to colonise the globe with fizzy drinks and Hollywood - you could do it like this: they had Star Trek, we had Blake's 7.

The difference between those two programmes says it all. On Star Trek, Kirk and his loyal, disciplined crew set out to export American liberal democracy across the galaxy - wiping out dictators here, freeing up markets there, showing alien peoples the virtues of the American way. "Why do you not kill me?" puzzled space-warriors would ask, as Kirk stood poised with his phaser. "On earth," Kirk would reply, "we have a thing called ... mercy" (or possibly justice or democracy, depending on the context).

Along the way, inevitably, the odd space maiden would fall for Kirk's charms ("Show me more of this `kissing', James Kirk"), but duty would call. It wasn't afraid to tackle the odd delicate issue - there were disguised allegories about the Vietnam War, for instance - but the basic thrust was optimistic: the giant Federation which the Enterprise represented was marching on to bigger, which meant better, things. The show ended in 1969, after three series which frankly weren't up to much in the ratings; but there followed umpteen films, umpteen TV spin-offs, and a vast merchandising industry.

Blake's 7 had many similarities. Blake and his crew would travel from planet to planet, trying to do the decent thing, waving the flag for democracy, freedom, treating each other with respect. But there was always an undercurrent of gloom, of optimism turned sour. This galaxy was run by a federation, too, but it wasn't big and cuddly: it was a ruthless fascist oligarchy (its motto "Strength from Unity"), out to crush the least whimper of dissent. Blake was an outlaw, framed on a child molestation charge. He only had five crew members and a computer on his mission, and they argued and bitched incessantly. Then Blake was killed, and his place was taken by Avon, a psychopath whose personal warmth made Spock look like Oprah.

And in the end, democracy and freedom lost comprehensively: at the end of the fourth series, in 1981, Blake turned out to be alive after all, on the planet Gauda Prime. But in a dark and confusing welter of mistrust, Avon killed him before being gunned down with the rest of the rebels, betrayed to the Federation. No "boldly going" here: instead, we got the boot stamping on a human face which George Orwell offered as a vision of humanity's future in Nineteen Eighty-Four.

Perhaps we ought to resist the temptation to see this as a peculiarly British trait. Something of the same cynicism can be seen in American science-fiction - the scrubby pointlessness of John Carpenter's Dark Star, the industrial squalor of Alien. What's so very familiar and patriotically heartwarming about Blake's 7 is the way sheer cheapness infused everything: a spaceship wobbling uncertainly across a cardboard sky, sets that swayed when anybody leaned on them too hard, and alien cities concocted out of a few yards of PVC in a gravel pit in Surrey.

And this lack of money pursued the show beyond the grave: no spin-offs, no plastic dolls or scale models of spaceships for the kids. Nope, all Blake's 7 gets is a one-off special on Radio 4 this Saturday, reuniting most of the old cast in a jolly romp called "The Sevenfold Crown", in which Avon and the crew race the evil Servalan - the elfin-featured dictator with the rather impressive embonpoint, played by Jacqueline Pearce - for possession of an ancient relic which confers unimaginable psychic powers on the wearer.

In its low-key, low-budget way, though, Blake's 7 has had a pretty good afterlife. There is a fan club, there are conventions (for the 20th anniversary of the first episode, next month, fans are taking over a hotel in Stoke- on-Trent for the weekend - a project that shows a nice sense of proportion). There are several sites on the Internet, full of storylines, analyses, lists of bloopers ("This could go on for ever," notes one such), guides to the gravel pits of Surrey, and sighing appreciations of the personal attractions of Gareth Thomas, who played Blake, and Paul Darrow, who played Avon. They also include, not irrelevantly, debates on "slash" fiction, a genre which postulates same-sex relationships between fictional characters.

Blake's 7 has acquired a credibility and popularity Terry Nation can never have expected when (so the story goes) he invented it on the spot, in a fit of desperation after failing to sell any other programme ideas. Brian Lighthill, producer and director of the radio version, who also directed some episodes on TV, thinks it is the interplay of character that makes it work - Servalan and Avon's love-hate relationship; Avon and Vila's hate-hate relationship. But I don't think that's it. I think it's to do with the sheer crappiness of the series and the crappiness it attributes to the universe: it is science-fiction for the disillusioned and ironic - and that is what makes it so very British. That's what ensures there is a corner of Gauda Prime that is forever England.