I found the cause of my problem:
It was a dodgy jack socket!
I have a few knocking around and it turns out I mixed up the 1 I took off the guitar with an old 1 I had lying around that didn't work, and I didn't realise :roll:
I haven't got any strings on the guitar yet, but i tapped them with a screwdriver and they all work.
So I'm off to re-string it and give them a try out! :D