Hi, hk, we've a visitor too, he's trying to move in, Sheilarose dawnf and emmmylou are right, talk to your vets. As far as i know, most vets have special set up for looking after strays/ferrels, they are sometimes supported by RSPCA and or provide discounts for ppl struggling money wise but if it's not your cat I'd be surprised they would charge you much, if at all. I can't swear to this as I've yet to take a found kitty to the drs, although I am considering checking our visitor with the vets.
However, I have taken several wild birds for treatment, ones our sol has brought me, she likes to give me them live, plus I have also taken a sea gull to the vets. I found him trapped in some plastic ring beer wrapping, got a freind to hold the bird whilst cutting it free and hold it in the car. I took the bird to be checked over, it was fine, had a feed and drink, they kept it over night before releasing it. I've never been charged by our vets for the birds treatment.
Best of luck, whatever action you choose, I hope your fur babies are ok.
Sent from my iPad using Tapatalk