Quick update - I don't have gallstones, but I do have raised LFTs (I'm guessing the azathioprine!) and my CRP is 34 ... so quite high really.
Their advice? Get my blood re-tested in January at my GP.
So I still have no idea what caused the deathly pain Saturday night, and seemingly I don't even know if I have UC either anymore!
Still, tis the season to eat, drink and be merry, so that's what I'll do!
Thank you all for your support!