Troublesome lights

I was towing a caravan to Devon this weekend and a light came on the dashboard telling me the transmission fluid was too hot. Its the red gear wheel with a thermometer inside on the right in the photo.
I managed to keep it under control by easing off when going downhill when the light would go out but it came on again when the engine was pulling hard uphill, there are plenty of hills in Devon.
Its booked in for some other work obn Wednesday so I will ask them to check it out. Landrovers are great when they are working but expensive when they aren't!