Fixing Dimplex oil filled radiator?

I have two Dimplex oil filled radiators, not sure the exact model but I think one of the Mk1 range (possibly the oldest). They heat wonderfully fast but only on y=the highest setting, the cut off as soon as they rech temperature and do not come on again until cooled off. I have found spare parts at electricspares, but I am wondering whether I need to change the entire thermostat and so, how do I do that? Many thanks, Cristina