I have just seen this film, and I thought it was rather sad that it got such a low vote score (3.9 as a "weighted average", at the time of writing) and a negative review as the only review it has on IMDb.
The film is obviously low budget and no classic. Having said that it has many plus points. It is well plotted and clever, with twists and turns (especially towards the end) and it sustains the interest well. It makes all the characters interesting, even the supporting ones, such as Peter Gilmore's flippant scientist. It makes a wise move in having June Thorburn - much underused by the British cinema - as its leading lady. There is also a surprisingly good supporting cast considering it is so low budget. And so on...
At an hour and ten minutes, the film is long enough to tell a proper story well, and short enough not to outstay its welcome. It provided me with an enjoyable time viewing it.