200tdi defender overheating?

Temperature gauge on my 200tdi defender 90 has been reading high, sitting just before the red zone. Needle moves up when I put the heater on which is strange. Heater works fine and engine seems to run as it should. Top hose feels hot, but not too hot that you can't hold your hand on it, bottom hose is luke warm.

I have changed the thermostat, radiator, water pump, temperature gauge sender, and back flushed engine. Can't see it being an airlock as 200tdi should bleed itself and its been running a while like it. Running out of ideas as what it could be. Starting to think it may be electrical, any ideas much appreciated. Cheers.

Check and clean the electrical earths. Particularly the earth strap from engine block to chassis/battery.

Yep sounds electrical.

Is it an original fitment 200tdi or is it a transplanted disco engine or something?

It is original 200tdi defender engine

I have ordered a second hand original temp gauge to rule out a faulty gauge. If that doesn't work I will do what Retroanaconda says. Thanks for replies.

Ditch it and get a mechanical gauge. One less hint to worry about

Fitted a VDO gauge and sender. Got to be an earthing problem somewhere. Gauge sits at 90 degrees C when warmed up which is about what it should be, but when I put heater on, windscreen wipers or lights it goes up past 100. When it's running without anything electrical switched it on its spot on. Can anyone tell me where the earthing points are that could be faulty. Cheers.