I would hope that the RSPCA would send someone who understands cats, so understands that cats don't know that being treated is for their own good and would be pleased that you are looking after him so well.
When I adopted my Jenny (RIP) she had recurrent ear mites, nothing seemed to get rid of them until the 4th round of vet-supplied drops, which must have been strong as it seemed to really sting - my lovely cuddly sociable puss used to dive under the bed and sulk for a good 15-20 minutes after application. The first time I put the stuff in, she did just that - and there was a knock on the door "Hello, I'm X from the RSPCA (shows card), just come to see how Jenny's settling in" "Oh, she's here, she's fine ...." - cat sat under bed glaring at the world with ears back. Luckily Jenny emerged and given everyone loads of purry love before the RSPCA lady had finished her cup of tea. They let me adopt another cat 4-5 months later and no problems with that visit.
Towel wrapping: I've found it best to sit with the towel on my lap and turn puss upside-down - a few tummy tickles before starting wrapping sometimes don't go amiss ;P - then - you have to be *quick* - turn up the bottom of the towel over the back legs, then wrap one side over the front paws and under the back, then the other side, keep it fairly tight at the neck, although not too tight of course. Naughty Jenny went all limp on me one time and I had a panic attack thinking "Oh no, I've strangled my cat" - so of course I let go and she was GONE. Cheeky, I didn't fall for that one again!
