Saw consultant (different one to who they told me it would be on the phone, so had to do my history again, only without my notes). He did all the usual leg wiggling tests and made my hips pop incredibly loudly.
The MRI shows a cam on the femoral head consistent with FAI, but he couldn't get it to impinge when manipulating my leg. His conclusion was that it was muscular, and that my super tight hip flexors are the problem (and the foot injury compensation idea is a red herring, it was just the straw that broke he camel's back). So yet more physio, this time with the head guy at the Nuffield Orthapedic Centre who is interested in FAI and hips in particular. Dunno when that will be though, or if it will amount to anything.
I've stopped riding completely as getting 15 minutes into a ride, everything tightening up and having to stop was too depressing. The turbo is packed up for now, the garage is getting a long overdue clear-out, and I'm thinking I'll stick a project car in there instead.