The quartermass experiment is not just a classic of british sci-fi - it's one of those things that entered the mythology of british fendom, which at this time, when most Doctor Who fen don't even remember Bertie Basset or that the Master was a furry at one point, means that I implore everyone who has access to the BBC iPlayer to watch The Quatermass
I'll be frank with you; The first is not particularly good. It's basically a competent version of Night of the Blood Beast
but set in london, and utilising the fine stable of RADA Actors who managed to get A*s in all their "staggering around and leaning on things" classes. Quatermass is unlikable and a dick on the scale of Ten at his worse, and most of the plot is conveyed via telephone and never quite managed to be tense or exciting (after all, the alien menace would, if left for a week or so, be very easy to deal and not be all that threatening anyway).
Nice use of a octopus though.
It's notable to a modern watcher because the zoos are abominable (tiny cages and all the animals pace back and forward like abused bears) and the finale relies on the fact that back in those days, TV shows, if they were recorded at all (the original quatermass serial wasn't, though that's not that great a loss by the looks of things), were recorded from the live performance rather than pre-recorded - though one is, I assume, not supposed to think too hard about why the BBC is broadcasting a live documentary about the renovation of an abbey at 8 o'clock at night.
Quatermass 2 however is what went down in fandom history - politicians are melted, slightly dumpy men attempt to be action stars while wearing gimp masks, "Sid James: character actor" makes a contribution to the plot, corpse robbing occurs at the slightest excuse, awesome special effects, graphic portrayal of an ordinary st. patricks day in rural britain, and of course social commentary involving the insularity and paranoia of wartime britain getting turned on the communities that rely on it.
And the finale is the classic doctor who scenario of a bunch of people getting stuck in a small room surrounded by baddies, with one coward who wants to just do what the evil guys want, an angry guy who's confused and/or drunk, some assistants and The Professor who knows the plot - and they have to save the world, defeat the bad guys, avoid being eaten by a hideous monster and if they can't do it all in ten minutes they're all going to be blown up anyway!
They're a very interesting counter point to the notion that once-upon-a-time british sci-fi was this super classy thing base around ideas and drama rather than explosions (even if Wells' War of the Worlds
, which was explicitly taking its cues from the scenery destruction pr0n of the less skiffy invasion novels of the period, kinda already does that).