Sadly, the original supplier of my Brompton is no longer in business (Ben Hayward and Sons), so I took it to my nearest dealer this morning. I was told the hinge bolt had sheared off and it isn't covered under warranty as the hinge itself is a wear and tear item, unlike than the frame itself.
On the plus side, they've just told me it's fixed so I can collect it tonight if I wanted. On the minus side, both my rims are badly worn (which I had suspected), so I'll probably have to take it back in to them soon to get a new wheel built. Back on the plus side, the hinge bolt didn't wreck the hinge itself which could have meant a new rear triangle apparently.
So, could be worse, could be better. As it's my main mode of transport that gets me to work and back it will have to get done soon, and it's still cheaper than running a car. Cheers for everyone's input on this thread.