Brian Donlevy reprises his role as Professor Bernard Quatermass from The Quatermass Xperiment (1955), making him the only actor to play the character on screen twice. In spite of this, he was reportedly "Quatermass" creator Nigel Kneale's least favourite actor in the role.
In the shot revealing that the Police Commissioner has 'the mark' he is consulting a typewritten checklist. Closer examination reveals this text to be the repeated phrase "Now is the time for all good men to come to the aid of the party".
When bodies are used to clog the oxygen pipe, the blood could not drip in the control room. The distance is way too far, the pipes have bends, plus it would be in reverse direction of the oxygen flow.
When Quatermass pulls up at the traffic lights in London a car to his left does likewise, but this car is not seen in the following close up of Quatermass at the wheel.