Thanks Desley.
She's seems very uncomfortable still, and occasionally howls. She's incontinent, but I'm keeping her in a large carrying cage and just keep changing it, so she doesn't have anywhere else to 'go'. I'm doing this because she's behaving like a cat who's just coming round from a GA: moving drunkenly, can't get onto her feet, and kind of crawling like a baby trying to stand. These could be side effects of the Milbemax, but she doesn't seem to be getting better.
I first noticed her symptoms yesterday lunchtime. I'd given her the pill at about 5pm the previous afternoon. I'd hate to think I'd killed her by giving her this; she was fine before hand. I knew vets were bad for cats! (me having a rant) I might persuade a friend to come with me to the vet today for support.
I'd always hoped she would go gently in her sleep, not howling and acting weird.
I think I owe it to her to wait till the end of the weekend, or longer, before making any final decision. It's a choice I'll have to live with, and I want to be sure there was no way back.
Thanks Ela, now I'll definitely give her time to heal, but will see a vet I trust. There are two of them at my normal vets.