It's most likely to be low down beside the tank (or maybe even under it if you are unlucky!) but you might need to trace the hose or the wiring to find it.
They are pretty cheap but they can be a right pain to replace unless you can get the exact same model.
Check pressure rating, if you have a calorifier make sure you replace with a lower pressure than the pressure relief valve or it will just pee out. EDIT: That is assuming it's a pressure cutoff type and not one of the small inline ones where the tap is just a switch.