thanks for the comments
We're not sure exactly what caused the bladder problem - 2 options seem most likely.
1 EC - E-Cunniculi, a parasitic infection of the kidney (can also affect other parts of the nervous system and the brain which can lead to lead tilt amongst other things).
He tested positive for the parasite via a blood test - although he may only be carrying it, it is apparently difficult to identify if a bunny is actually suffering from it.
The EC being active in the kidneys can lead to renal (kidney) failure, which buns can hide pretty well.
The EC could also have affected the nerves that trigger the 'I need to pee' instruction which in turn meant he was retaining urine.
When he was ill before Christmas, he bloated pretty quickly. It turned out this was his bladder which went up like a balloon. The vet expressed him, but it may be that this incident stretched his bladder walls and is what lead to his bladder being very weak now.
2 - that the renal failure was caused by a back up of urine, possibly due to something like Cystitis. The bladder bloated due to it being too painful for him to pee which caused the bladder walls to weaken. Urine backing up in his system would have damaged his kidneys too.
We're not sure if its either of those 2 possiblities - or even something else. But he's on treatment for EC (which can only at best stiffle the spread of EC - it cant cure it) and we're expressing him twice a day.
He seems to know there is something wrong with his kidneys because he drinks like a fish - which I guess helps to flush as much out of his system as possible given his kidney problems.
The one upshot of this is the time he spends sitting on our laps. We have to get him 2 a day, and he gets lots of attention and cuddles during this time. He gets plenty of attention the rest of the time too as Pam works from home - but you cant beat having him on you lap for a while
thanks for the feedback
Martin