Hi Lyn, thanks for the info. Yes, mine is/was a true anal cancer, a very little bit of the lesion poking out of the exit point and making sitting down and wiping after motions uncomfortable, and the rest of it parasitically growing into the wall of the anus above it.
The surgery removed all of the cancerous tissue, along with 14 lymph nodes; there was no sign of cancer on the outside of the histology sample. The prostate cancer is ongoing, but seems to be contained by the Prostap jabs (next one next week), so it was almost certainly not a direct spread from that.
The good news is that (as promised) having it removed has cured my piles!