A quick update. I started taking the tablets at 5mg, but was getting symptoms like sinusitis - made wearing my glasses painful and was having to take paracetomol for the pain so I could sleep. After speaking to the SCN I was told to cut the tablets in half, which seemed to work.
However, at that point I started to go downhill - having to get up every two hours in the night, losing weight, feeling fatigued, so I stopped taking the tablets. That didn't help and it became obvious after another couple of days that I had an infection. A dose of antibiotics cleared that, but it seems that the infection had inflamed the bladder, so I needed a few days and ibuprofen to bring things back to normal. As a bonus, it seems like the single problem I was having with my continence (getting a dribble sometimes when passing wind) has just gone.
Now they are back to normal I've started taking half tablets again, and apart from a slightly stuffy nose on the first day, I don't seem to have any side effects this time round. Wondering if I should start taking full tablets again...