In the absence of any further suggestions from agent or publisher, I decided to contact the Society of Authors. I had a very nice reply from one of their lawyers, saying it was a highly unusual case and she'd never heard of anything like it. She has asked for a copy of the contract and correspondence between myself and the publisher, but the trouble is the only emails I kept were those containing editing instructions. I didn't think to keep the ones that asked me to make the sex more graphic. If I had, I may have had a good case for... for what? I've had two-thirds of the money and can't expect to get the third I was to receive on publication..
I suppose all I can push for now is to get the rights back so I can publish the damned thing myself. Or to ask to be paid a further advance in order to rewrite what will in effect be a very different book, with all the smutty jokes and talk and raunchy references taken out, as well as the sex scenes themselves. Any views, anyone?