I picked up a BP EE headless body on ebay a couple of months ago for a bargain as the right knee joint was broken. It had been repaired with a small nut and bolt through the joint holding it together.
It was a pretty good fix and the leg was still in working order but I wanted something a bit more robust and permanent.
After much thought about what to use for it I had an idea of using a sprue from an old Airfix 1:72 set of figures. (The figures had already been removed but luckily I kept the sprue's)
So...heres my tip.
Cut a small lenght of sprue slightly wider than the whole knee joint and feed it through (the sprue I used was a very good fit but could still be pushed out again)
To make it a permanent repair heat up the tip of a small filler knife or flat head screwdriver, you don't need it red hot just hot enough to melt the ends of the sprue. Carefully rub it over the end of the sprue each side of the knee joint making it nice and flush with the joint. This also widens each end of the sprue to stop it moving out of place.
Finally I gave each side a very light rub over with fine sandpaper to give a nice smooth finish.