Repton (well, the Repton series -- Repton 3 is the best of them).
Don't know if any of you have even heard of this game? It's very old; I was playing it on my BBC Micro when I was around five or six, seventeen years ago that is. Absolutely brilliant though.
If anyone's curious at to what it looked like, my Repton Cheapo style is nearly ready.......
Incidentally, that computer died around twelve years ago, but last year I finally managed to complete Repton 3 when I came across a remake on the Internet. One of the happiest moments of my life.