Disco 2 Transmission overheat light problem

I have a automatic V8 4.0 Disco II, The transmission overheat light comes on almost as soon as I crank her up, it started coming on about 18 months ago in hot weather/stop and go traffic, it would go out once I got up to highway speeds, I had the tranny checked and serviced, they found nothing wrong, then it started coming on in all weathers after about 30 mins driving, but as I've just said it now comes on almost immediately, even before I've driven out of my driveway, does this sound like a serious problem with my tranny (she shifts just fine and no excessive heat coming from the consul) or is it possibly an electrical problem causing the light to come on? thanks in advance for any input.

Most likely thermal switch on its last legs....you will find it screwed into the side of the transmission cooler out front of the radiator

Thanks.