Just above the bottom hose spigot on the radiator in a plastic housing. Have to buy the complete unit part no. PEM101130, around £40.
 
i replied with something considerably stupid and i would have looked like a halfwit, i didn't want this to happen so i tried to delete the reply but didn't know how so i edited to the above.

i said "is it behind the radiator at the bottom" but that would be a stupid place to put the T-stat!!!! there now i look like a halfwit!!!!!

EDIT: turns out i was right, now i look like a know-it-all!!!!!
 

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