acrid smell from dimplex xt24 series h heater?

After the heater has been on for 3 hours or so a very unusual smell appears to be coming from the above unit. It has given twenty years of trouble free service but now has been turned off. The smell is unlike anything I've experienced in forty years as a radio engineer. My guess is that it is the insulation vapourising around the thermostat that I have seen flashing through the top grill.
Has anyone experienced a similar problem/smell?