Ian
Like DM's OH I have been prescribed solifenacin. I have had numerous urethral Catheters,the first one probably the worst. From my experience do not cut down on fluid intake it did not help me, avoid getting constipation and wind if possible, it all squeezes on the bladder. Have you also had bypassing where urine comes out between the Catheter and the penis, this is apparently caused when the bladder squeezes it is trying to force urine out,but the outlet of the Catheter is above the balloon so the only place it can go is between the Catheter and urethra. I found sitting on the loo passing a motion was pretty uncomfortable.Bending forward also caused the spasms to start. Is your Catheter draining well, debris can restrict the flow but you will be amazed what can come down a Catheter.
Good luck with the removal and Twoc, I always drink plenty before my twocs and stay calm.
Thanks Chris