Where the plastic pieces of the control valve meet the metal fitting of the valve body there will be a circular collar, usually plastic - unscrew the collar and the TRV top will pull off.
Heres a pic...
http://lpssupplies.com/images/TRV2_B.jpg
They unscrew anticlockwise so looking down from the top where you stand it will need to be turned clockwise.
Good luck...
Peccavi
March 2009
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