Just back from the vet with good news.
He took three X rays, as below, and decided that the blockage is contained within the intestines, but there was no torsion, so we were pretty sure it was impacted faeces, not a tumour or a twist, making it possible to consider giving him oral laxatives.
We were just discussing laxative possibilities (anyone have an opinion on Mineral oil or Lactulose laxative in snakes?) when he started to pass urates.
That was messy but good news, but then there was a little blood in the urates, and a visible inversion of rectal tissue, and he passed a decent size stool, soft on the outside but dry and impacted on the inside. I've rarely been so happy to be pooped on
So, it's pretty obvious he had impacted faces for some reason, and we are hopeful that with the help of some mineral oil, which we are trying to source now, we may be able to shift the rest of it. The remaining lump is much smaller and softer and is nearer the cloaca than it had been, so we will continue with bathing and handling to keep him moving, and wait for another stool before we try feeing again.
I'm sure we are both much happier this morning - 3 - 4 months of constipation can't have been nice for him.
(Please excuse my rings in one of the X rays - the local vets aren't too thrilled to hold a snake still, so I had to do it myself)