im now 10wks post op tkr and have been fine up until now when go to bed i seem to wake up in the night with leg aching like mad at the back of the knee why would this be and what can i do to help it?
Do you sleep with your leg elevated?
no
There is your reason for your pain. 
Elevation can keep most of your problems down. I had my right knee replaced in April of last year, and had my left knee replaced 4 weeks ago
Elevation definitely helps (I use pillows) and I ice for at least 40 mins before going to sleep.
My worst pain is in my thigh but I also get the posterior pain. GOOD LUCK
Stretch your leg out before bedtime by sitting back in one chair and spanning your leg across to the other chair - equal height and feel the stretch. Just do it for 2 minutes, rest and do it again. I bet it will help. Best wishes