During World War II, the Royal Enfield factory was moved to a complex of underground quarries in Westwood, Wiltshire. David Blasco has the full story in his blog. Production in the underground factory continued after the war all the way to the Interceptor engine. During the cold war the RE workers were unfortunate guinea pigs in a chemical warfare experiment.
I set out to find the entrance to the factory using satellite pictures. I think I found it, or at least came close to. According to this Wiltshire government site, the entrance is "north west of Upper Westwood Farm". The British Documentary "Top Secrets Revealed" has a picture of the entrance like it looks today,
and it also includes a map of the underground quarry, which places it south of Avoncliff (the grey portion of the middle is the RE factory, the darker gray areas on the sides are labeled "old works" and the appendage at the bottom is the place they stored the art pieces from various London museums during the war),
I'd say the map matches the wooded rectangle in this aerial photo,
which also appears to be "at the northwest end of a farm". Notice the little white dot in the dark green field to the west (more later).
Unfortunately it is not easy to make out what is going on there or where could the entrance exactly be,
Could the little white dot in the green field to the west, here magnified,
be this ventilation shaft (from the movie)?
Update 3/3/09: Looking at the same image with Microsoft Live Search one gets a much better view and it is clear that the entrance is not where I thought. If you go to the black and white map from the movie and look at the upper left corner one sees a line that zigs and zags horizontally. That is a canal (the reason it zig-zags is that the canal goes over a railway line in a sort of water bridge!). Researching around I found that the quarry had "access to the canal and the railway". That led me to search the upper left corner of the map, to which an arrow seems to point in the black and white image, though I cannot make out the text. I found this building, which is close to the canal and seems to involve an underground entrance. You be the judge! (Incidentally, the white dot IS the ventilation shaft).