I think the thermostatic control (143211 086-6 printed on the component) is worn out. The heater won't even switch on until the dial is up beyond the 4 mark and regardless of the external temperature, switches off after about a minute and won't come back on again for several minutes. I can't find anything wrong with the rest of the heater so I'm guessing the plastic bit that adjusts the bi-metal contact is worn out.
Where can I buy a replacement? I've searched rs-online and ebay without result but maybe I'm not using the right search. And if I can't buy a replacement, is there a near equivalent I could use? This one has spade connector terminals so I think it should be easy to swap the part out.