I wanted to pop back on and add my own update to this thread.
We are starting to feel cautiously optimistic about Lucy and her IC.
With strict diet controls and daily Cystease we are slowly cutting back on her pain relief and things are looking ok.
We didn't try Cartrophen injections becasue while my vet was looking into this we made one other change to Lucy's diet - water.
Twice a day she has a lettuce leaf or handful of fresh grass quite wet with filtered water.
I just pour water from the jug onto this and shake off the excess before putting it in her cage.
It's such a simple thing but it really does seem to be helping.
It's only been a few weeks so far, but at this point I am happy to go with whatever works.