Search function says 'acl' is too short so shut up.
My diagnosis is incomplete: I'm waiting for an MRI scan. But the general consensus is that I have a ruptured ACL.
Apparently this will never fix itself, and I'll either need a ligament graft, or I'll have to build up a load of muscle around the knee to compensate.
If you've had an ACL rupture or tear, post your experiences, many thanks.