I took Cally to the vet this morning to take a look at her tail and the news is not nearly as bad as it might have been.
The vet found a small puncture and some infection in the wound site so it's most likely that it's an infected bite from Princess. It *might* have been Pumpkin, but the odds are stacked against it. Still painful, but the treatment is fortunately exactly what I'd already done with the Metacam, thinking it was arthritic pain. She's had a shot of antibiotics and I'm to continue pain relief and wound cleaning with betadine.
It's a great relief that this was not a new degenerative stage in her general condition. She should bounce back gently and with a dignity suitable to her advanced years. Until the next time :/
Poor old cat.