My personal favorite of Hammer's, Quatermass Trilogy. Andrew Keir is outstanding in the role of Quatermass and is supported nicely by, Barbara Shelley, James Donald and Julian Glover. During work in a London tube station, skeletal remains are found and figured to be at least five million years old....a wonderful find in itself, but upon further digging, a spaceship is discovered and things become a bit unsettling. Insectoid bodies are discovered in the spaceship and it's determined that the insectoids are or were inhabitants of Mars and routinely made trips to Earth, enslaving the humanoids of that time as Mars was constantly battling on it's own planet.
A very well thought sci-fi effort from Nigel Kneale and wonderfully produced by Hammer and directed by Roy War Baker, if you see just one of the Quatermass films, make it this film!