An update on my (still ongoing) injury:
After essentially plateauing, and not getting any better my physio sent me to see a rehab specialist doctor who referred me for a CT scan. I had that a couple of weeks ago, and it revealed 3 injuries; the original post-tib injury, an Achilles injury (partial tear and poorly healed at the heel bone insertion point), and an old IPR tear which I had no idea I'd done.
The post-tib is not in too bad a condition, so they are going to continue the loading on that, but the heel insertion point is a cause for concern. I've been prescribed a course of shockwave therapy which is apparently blasting the area with sound waves to force the tendon into repairing itself in a predictable way. It's a 15 minute session, once a week for 5 weeks, with a 48 hour rest period followed by gradual loading. I've been told it's likely to be painful.