TOMT: When you can't remember that…thing…

When I was a youngish teenager (early 2000s), I read a book that a friend recommended and lent to me. I think it may have been part of a trilogy, but I can't quite remember. It was about a man from our world who finds himself in another (fantasy) world. I think he gets there by getting in a motorcycle accident, and he's flung from the bike and ends up in the fantasy world, and becomes a hero/champion there. It's sort of set in the UK, in that the fantasy world is roughly overlaid on our world, i.e. familiar places and landmarks in the real UK have corresponding places in the fantasy world. I remember the cover had a normal modern road map, wth distortions like drips of water, through which you could see the fantasy world map beneath. In the fantasy world I seem to recall there's a mechanical behemoth of some kind ravaging the land, which turns out to have been built by another person from our world who came through long before and decided to use his knowledge of technology, etc. to become the villain rather than a hero.