The Quatermass Experiment

Or [pedant]it is The Quatermass Xperiment[/pedant] if we are talking about the film, as this is.

As in these things matter, especially in 'cult'; in this case, that Hammer put the emphasis on the X certificate through their alternative spelling, that this was not something the audiences of the time were going to get on BBC television.

http://www.bbc.co.uk/iplayer/episode/b007y4fk/The_Quatermass_Experiment/