Heater Plug "On" Dash light

After fitting a new'ish warning lights display panel in the dash because the old one had all but worn away, the heater plug timer light has started to come on as it never did before,(I used to just turn the engine over when I thought they were hot enough)the problem is, it dos'nt go off after a time. Is there a timer or heat sensative switch which turns the light off when the plugs have reached there optimum temp and where is it?

The timer relay is beside the fuel filter, ye might just see it in this photie...



it's the black box just behind the filter housin'.
Does the light go off after starting?

Cheers Buster, the light goes out ok but only after iv,e turned the key to start the engine, until then it will just stay on as long as the key is turned to the glow plug stage.