The builder dug out a trench to lay the new foundations and found there were no foundations - the brickword was bedded on a scattering of balast about an inch thick. A couple more inspection pits around the perimeter confirmed this.
So it looks like we've got to knock it down and start again - which is an option we'd considered when we made an offer on the house because we thought it might be easier and better than working with what was there.
Nothing showed up on the survey - and I can't see how it would have unless the surveyor had been instructed to dig pits.
The council apparently knew about the extension as I checked before we bought the house and there had been a planning application which showed the extension as already existing --- but they didn't seem as if they'd taken any interest in it as it was done under permitted development rights. It was put up about ten years ago and obviously having no foundations didn't meet building regs.
Do we have any comeback against the seller (who incidentally is a solicitor) or is it just a case of caveat emptor?