The problem is now about 90% fixed. With the help of a relative that luckily used to work for Metro Rod thirty years ago (the things you find out in an emergency eh!) the leak under my kitchen floor has been capped. After five days and a bit more digging the water has eventually stopped gushing out of the house foundations, and my drive is now back in one piece.
There is still the issue of an unused active mains pipe hidden somewhere under/around my house, but getting Welsh Water to do anything about this is proving a real struggle. On one of their five visits to look at the problem Welsh Water told me that if I could find the leaking pipe then they could trace it by catscan. Well I have been phoning them for the last two weeks, but getting them to turn up seems impossible.