Note
Saw the physio (was Morag not Tracy though), and she initially wasn't sure but thinks it's my posterior tibialis tendon, and after some googling I agree. No idea how, but yeah. She offered crutches but they didn't have any the right size, plus I'd cycled there anyway (which was just about ok, don't have to bend the ankle too much). RICE for a day or so then got a few little exercises to start doing. Good job I don't need to leave the flat til Monday.
Depressing, thought I'd done everything right and was looking forward to the O season. No idea when I'll run next.