Appointment with James today. He reckons the pain is coming from an irritated tibia-fibula joint. The reason the joint has been mistreated recently would be due to a hamstring tendinitis where the muscle inserts into the joint and therefore pulling on it. And the issue with the hamstring itself can be due to downhill running or overstriding according to James. As this hamstring had been sore in the first part of January following (flat) races/sessions, I guess maybe overstriding then when I try to pick up the pace.
James massaged the hamstring and calf muscles as they were rock solid and after that I could actually walk better - the pain was still there but I think I was limping more because of the reduced range of motion (due to everything tightening up to protect the joint) rather than the pain.
James gave me stretching exercises for the hamstring and also one strengthening exercise, all to do daily. I can also foam roll the top of the hamstring and the lower calf (basically everywhere but where it's painful).
I have another appointment in a week to see how things evolve and also to make a joint decision (no pun intended!) on whether I need an MRI referral from the GP (the worry being that actual damage has occur at the joint location).
Regarding cycling, it has the issue of locking the hamstring in position and that's not ideal in the current situation but if I lower the seat and stretch straight after, it's allowed.
Reduced walking to minimum and shorten stride.
Swimming 'as able' - the breaststroke is not ideal because the lower leg sort of twist but unfortunately I don't know how to swim the front crawl. So probably no swimming from me for now.
And finally, no glutes exercises for now as it might pull on the hamstring.