I have a feral, Chubby, who sleeps under my bed, alongside 5 resident domestic cats. Chubby is in/out but is unhandlable, although he is on fair terms with the furry gang. I give him a Capstar tab every few weeks, crushed in his food, to clear any fleas, the rest of the clan are on Frontline topical, so this protects them, the house and Chubby and does seems to work really well.
Capstar is available on-line from VetUK and many other outlets, but it doesn't provide any ongoing protection for Chubby, just kills the fleas on the cat at the time of treatment.
I also use a flavourless granular wormer in wet food that seems to be palatable. This means I can keep up with Chubby's welfare without the trauma of trapping and vet visits.
It works for us, but may not be everyone's preferred method.
