OK, I think it's time I gave you all an update!
So I spent a few happy wombly hours in the workshop melting bits of metal together and generally creating mayhem, and then set up the trail-cam and waited......
Happily, by the next morning I had caught myself a feral cat!

Unhappily, once caught, he was the wildest, sharpest ball of hate and muscle I'd ever clapped eyes on!!

However, he soon calmed down once he realised we weren't going to hurt him, and over the next few days we even got to the stage where he'd take pieces of chicken from my hand (so perhaps not completely feral then?).

He had his balls removed last week (paid for by the Cats Protection League, which was nice). However, although we kept him caged afterwards in an attempt to tame him up, he's actually become far more aggressive and will now go for any hands / spoons etc that are poked through the bars.
So, what do you think folks? Should we just let him out now and see if he hangs around? I reckon it's probably futile to keep him caged any longer, since we're clearly not going in the right direction, unless any of you can advise me otherwise. Does anybody know how long it will take after neutering for his hormones to calm down?