Just out of interest, United Utilities have delivered a free tube-thing (don't know the proper word!) and metal cover, and the gas men have very kindly agreed to place it in position and assured me that they won't tarmac over it.
I went out to measure it and the tap is about 32" below the (proposed) road surface, so it's not going to be out of reach even without a long-tool-thing (don't know the proper word either!).
Unfortunately, there's no pavement and the tap is in the road (about a foot from the edge), and it's a busy road. So I've decided that if disaster strikes and I need to turn it off quickly, I'll park my car in front of it (with hazard lights on) for a few minutes whilst I turn it off. No yellow lines, fortunately.