Dominic - I think you're probably right about gait being the cause.
In Leeds I massively recommend Coach House Sports Physio on Otley Road. If they're good enough to work with Jess Ennis, the Brownlee brothers, Charlene Thomas, Kelly Holmes and...er...me...
They'll assess and work on treatments and if podiatry is seen to be an advised route they have one of them too but they'll only refer you to them if absolutely necessary. i.e. if its based around gait or strength they'll fix it that way rather than use orthotics as a sticking plaster remedy.
As an example, I was due to be running Amsterdam next weekend, got what I naively thought was a recurrence of achillies trouble from last year and rested for 2 months with little improvement. Took the CSP guys about 2 minutes to diagnose something quite different, massage it, give me some strength drills and I'm back running again.
If you want to lnow anything more on them send me a message.