In reply to r0b:
Thanks, now I know what it's called I can do a bit of reading around it.
Looks like it's caused by a bone spur or ankle arthritis - both of which sound quite worrying.
I see both of your suggestions are around improving flexibility in the joint. It feels to me that this might be the wrong way to go, as if it's more flexible it's more likely to crunch into itself. Am I missing something?