Thanks Karen. I saw my family doctor today, who did a quick anal scope (I think that's what it's called) to rule out hemmerhoids as a cause. No hemmerhoids at all, but she could very quickly see an ulcer that bled when touched with a swab. She has referred me to a GI specialist for a colonoscopy, which should take place within a 2-3 week wait, and said that it may be "proctitis" . . . only the colonoscopy will tell for sure, of course, but looks like you may not be that far off!
Thanks again for your advice.