Just an update: I found this video http://www.mobilitywod.com/2012/12/knee-pain-got-full-knee-powerrange-of-motionpotential-terminal-knee-extension-part-1/ and it seems to be helping.

If I do some stretching or even stick a lacrosse ball under my knee and flex for 30 seconds I find at least temporarily my right knee is close to being fully extended.

Also prior to the stretching I wasn’t very comfortable getting up off the ground on the bad side, but post stretching there doesn’t seem to be any cracking knee etc.