If you haven’t seen it…
The Quatermass Xperiment was an adaptation by Hammer Films of a very successful 1950s BBC TV show. It is the story of an astronaut who returns to Earth bringing with him an alien infection that threatens mankind.
In the USA it was released as The Creeping Unknown.
At the time it was regarded as a horror movie as much as a sci-fi movie, and it paved the way for the Hammer horror films that brought the studio much success.
Trailer
***SPOILERS BELOW***
Plot

“You can’t park there, mate.”
A rocket ship crash lands in field in England. It is the sort of crash landing where the ship gets buried nose-down into the soil (what I call a science-fictional crash landing) rather than, say, blown into a million pieces on impact (a “realistic” crash landing). It is a British rocket ship returning from space. The only survivor is the catatonic Victor Carroon, who is taken to hospital. Believing Victor is being held prisoner, his wife arranges for him to be rescued. Once free though, it becomes apparent that Victor has been infected by some sort of alien life form, which makes him violent, and able to “absorb” other creatures. After absorbing animals in a zoo, he turns into a huge alien monster. Professor Quatermass leads the hunt for the monster, believing that if he does not catch it before it “spores” then the alien life form will take over the world. Eventually Quatermass corners the monster in Westminster Abbey, and electrocutes it, killing it.
Comment by Bobby
I watched The Quatermass Xperiment for the first time last night. Oh Em Gee, it’s Doctor Who and the Seeds of Doom – my favourite ever Dr Who adventure, the Tom Baker/Elizabeth Sladen story about a man who gets attacked by a seed pod then slowly turns into a giant green tentacley monster (called a Krynoid, fact-fans). I have always thought it was the pinnacle of horror/sci-fi storytelling, but now I have discovered it was a rip-off of an old 1950s B-movie. I have to go and lie down now.

Space octopus/Westminster Abbey for some reason not as iconic as the King Kong/Empire State Building pairing.
Main Cast
Brian Donlevy … Prof. Bernard Quatermass
Jack Warner … Insp. Lomax
Richard Wordsworth … Victor Carroon
Margia Dean … Mrs. Judith Carroon