I second the microchip catflap.
My mum had had the magnetic ones before and the cats always managed to push their way through, no problem at all with microchip, in fact even we can't push our way through it (though could probably kick it in if determined). Best investment we ever made for the cats. The one we have is the one that is powered from the mains.
I agree that a cat without a collar doesn't mean it isn't owned, some cats just won't wear a collar no matter how much an owner tries. But the owner can microchip the cat, so if you do catch him get the vet to scan him. In addition it is true that cats independence is part of the attraction, and if they decide they want to live elsewhere they will up sticks and go. If mine made that decision I'd rather someone took them in and loved them than turned they away in case they already had an owner. Luckily I get to feel blessed each evening mine return through the flap knowing that my home is where they call home, for now.
Dans