Any ideas on what is causing overheating in 1996 defender 90 tdi 300, it’s usually ok for 20minutes but then will shoot up from 60 degrees to 110 in a matter of seconds (or by doing more than 40mph). I have checked for coolant leaks which seems to be fine.

Could be a few things.
The thermostst may not be opening.
The EGT is high if the boost and fuel has been 'turned up' too far.
The viscous coupling on the fan isn't 'coupled' and the fan isn't 'cooling'.
The temperature sensor isn't telling you the truth or is possibly not matched to the temp gauge so the readings are not true.

When it gets to this high temp, how long does it stay that way ?

It should be very obvious if it is overheating not just form the gauge, there is usually a nice hot smell, and if you pop the bonnet everything is properly sizzling. is this the case when it is overheating for you?

I would start by checking the viscous fan as that is an easy test. next time it overheats turn the engine off and see if you can spin the fan buy hand. if you can the viscous joint has failed and will need replacing. when it gets hot the fan should be locked and unable to turn without turning the belt.

a second test would be to remove the thermostat and run like that seeing if you still get overheating. in this temperature ethe fact that you are running open circuit through the rad will not matter. you can also test the thermostat with a pan of water heating up on the hob with the thermostat in it and a thermometer. watch the stat and when it opens check the reading on the thermometer.

Everything else @miktdish has mentioned are good suggestion but the two above are the easiest and quickest to check as a starting point.

I would be investigating the head gasket, does it use any coolant?

That 60 to 110 in second's is pretty much physically impossible

Given that figures are quoted it's an aftermarket fitment gauge

my 2c

Check your actual engine temp with a IR temp reader to ensure you are not actually cooking your engine.