Kind of slightly in the same boat as yourself. Ive got osgood schlater disease in my right knee. Its got much better after ultrasonic treatment but when riding it can get painful. Once i give it a quick stretch its ok....but preferably im thinking some kind of support which might help with the knee not becoming as sore.
He wants £500..........seems dodgy that for a bumper.....though cash deal he see's it as doing you a favour. but as above check he is legally allowed on the road. Unless it was something like a fancy car he hit and it fully requires a new bumper £500 seems expensive.