Bless, did the vet say if he had been neutered, if he smells like you say it sounds like he is spraying.
Celia Hammond may be able to help - they deal with strays
233-235 Lewisham Way,
Tel (Veterinary Clinic): 020 8691 2100
this is a bit on their site about neutering male cats:
To keep your pet from getting lost
When mature, your unneutered male cat will begin to roam further afield in his search for female cats. As he travels further, he will be at risk when crossing busy roads and from fight injuries acquired during territorial battles with other unneutered male cats. Many male cats roam just a bit too far and cannot find their way home. They then become one of the UK's 2,500,000 strays that struggle to survive on the streets. A large proportion of cats that are reported to, and taken to, rescue centres are tame unneutered males once owned pets.