Idea 1: it sounds like lateral meniscus tear.
Idea 2: stop leg work. You are clearly aggravating condition. If it is lateral meniscus year you could cause further problems (bakers cyst development, further tearing, muscle tearing)
Idea 3: save £60 get your physio to refer for a private MRI. £300

Money well spent if you want to be healthy.

Regards,

Niall