the most likely issue with it is the heater element, these can be bought in ebay,to check the existing element which is L shaped and attached to the pipework on the back, by removing the 2 attached wires and checking it with a meter for continuity will tell if it is faulty or not, if its element is heating this will exclude the stat which rarely goes faulty, which would then suggest the gasses have split, hopes this helps you somewhat.
keymatic Irl., August 2013