I went to the incontinence clinic today finally. I apparently have a flaccid (passive) bladder with very high residual volume (about 400ml out of a total capacity of about 750ml), and possibly a paralysed urethra, no obvious cause yet. So I'm being referred to the hospital for more intensive tests. Yay (not) for more NHS waiting lists.
The nurse also gave me a useful tip for improving the performance of a diaper. She said not to wear them straight out the packet. Instead, take out all the ones you're going to need for tomorrow, open them out and give them a bit of a shake, then put them in a drawer without squashing them too much. This fluffs up the filler and makes it more effective.