For Tony - Fitting the thermistor?

Hi Tony, thanks for the advice re the thermistor. I have purchased said item from Maplins. Am I right in thinking that it is the white ceramic block screwed to the underside of the boiler - blue leads on either end (but looked more like a bi-metal strip when I got into it?). There are only two other items on the undersideof the boiler. A relay with three purple isulating discs and a single pole sealed brass item that is fiited and sealed into the boiler housing. I would be most grateful if you could advise.