I couldn't just ignore it as if they are scammers, they are probably contacting other rescues etc and if there is any chance they are looking for free "baiters" or other dodgy doings, other cats could be in danger. I would imagine this is one the RSPCA should investigate. If they are genuine animal lovers, they will understand. Hard for you to do as I know you are up to your neck at the moment. Might be an idea to string them along and see what happens. Isn't it unusual for people to ask for a random cat?