Thanks guys...
It's developed over time Gerry, in the last 6 weeks to be precised. He's been eating the usual catfish pellets, the same as the others.
There's no effect on his bouyancy, which I'd expect with an air sack this large, however it's virtually translucent, doesn't inflate/deflate with gill movement or eating, yet looks like it's about to 'pop' at any minute. It doesn't appear to affect swimming or eating and he sifts through the sand with his other buddies as normal, in fact head on he looks like he's pearched on a space hopper (remember those?).
Epsom salts, ok.... but will salt be safe with corys? And will ES affect the rest of my tank's inhabitants?