Really sorry to hear this Jackie
I've only just caught up on this thread.
Have they said for definate that it has gone to full blown FIP or just showing signs of exposure to a Coronavirus?
If the poo sample came back clear... ask them to re-test for Trichomonas. (spelling!?!) You have to specifically ask for the test as its not done as standard. Cost for the test is around £40 (well it is up here)
It seems to be rife with certain breeders and can show up no signs on a regular poo test but can be mistaken for FIP / Coronavirus .
Just about every cat probably has a coronavirus antibody level did they tell you what yours were fro the bloody tests?
Treatment for this is Metradizonal (spelling?!!)... but Stomorgyl contains adequate doses for 10-12mg per kg. A 10 day course of this will start to show signs of improvement if it is Trichomonas. Maybe worth speakign to your vet to see if that would help??? could be worth a try?
We've been down this road with rescued pedigrees quite a lot in recent years.
Good Luck with tests, etc. Really hope that they start to show signs of improvement soon.
Sxx