I did the opposite and nearly regretted it. My then DH told me not to let in or feed the cat who was covered in sores and hungry and who had been coming in for a meal and a cuddle as he was sure she was owned by eighbours who had 3 or 4 other torties. To cut a long story short, a few weeks later and having not seen her for a week or so I got talking to someone on my dog-walk route. He too had been feeding her and advertised her as found to no avail so had taken her to the cats home.
I quickly dispatched DH to the cats home and after some weeks, during which she was treated for a terrible flea allergy, Jackie became mine. Some months later I discovered that before I had moved to the area a family had done a runner from repossession agents on Xmas day and had left Jackie behind... it was snowing throughout Xmas and of course the poor mite had survived on her own through a bitter winter. I wish now I HAD just taken her in at once but of course she could have had a worried owner like you.
But this isn't helping you, so a few more suggestons might. You say you have advertised online and have no doubt tried local rescues - but have you posted on any local rescue forums or asked the rescues if they would put a post on their websites?
Also your local paper? They MIGHT run a small article if they are a small publication or if not you could put a classified ad in (our paper offers a week free for lost pets, so its worth asking if yours will). Likewise your local dog rescue - if they have a forum you could post there. Don't laugh but most dog owners are daft about most animals and will be happy to keep an eye out for you and spread the word.
Also try the Yahoo email discussion group UK ANIMAL RIGHTS. We members have contacts nationwide and will gladly email your request on to others in your area.
Other than that, advertise and keep advertising... pet shops, post offices, supermarket notice boards and the like. I have known of dogs to go missing for ages which have been found this way so see no reason why a cat should be different.
Don't know if you have tried already but this is worth a go too:
www.nationalpetregister.org/
Good luck, thinking of you.