Thanks for the great advice. Ear-tipping it is. One of your contacts, Dawn, offered help with finance, which should help me to keep the females for a few days.
Thanks for the great link MM, have bookmarked them all.
I may be changing my vet to one more accommodating who will do the ear-tips and has a less jobsworthy attitude. I have a poor memory, especially for faces, so wouldn't remember individual cats who've been in my care for a few days/week. Because the problem here will be sorted on an occasional basis (in June, there's always a new batch of strays turfed out by students, usually unneutered), no way will I remember who's who. Also, my current vet spays underneath, which makes identification of spayed females difficult.
Now I need to get the cages, organise transport, get a good vet, and evict Fred and Victoria from the back room. Fred is turning into a wonderful character. He played with my toes - which I wiggled for him - yesterday, and ran off with my specs today. Little monkey. Polly is in love, Sam is unimpressed.