Easy to replace the top of the valve - might also consider turning off the outlet valve (lock shield valve) to restrict water flow. Thake the cap off the valve at the other end and use pliers to screw in (clockwise) the valve stem.
Wait a couple of hours for the rad to cool - than open the lock valve half a turn at a time to get the temperature that suits you.
A new TRV is quite cheap - if you buy the same model you may well get by by only changing the top piece. Just undo the plastic collar at the bottom and lift it off. It may be partially undone already and that's why your rad is out of control.
Good luck...
Peccavi, March 2009
