200 tdi overheating?

Hi just got a 110 with a disco 200 tdi in it. The problem am having is if i run it with a thermostat in then it go's to 3/4 on the gauge and my radiator is cold as is the expansion tank. If i run it without a thermostat then it go's just above the white (eventually) on the temp dial, radiator is warm so is the expansion tank. I got a new thermostat 88c and have tested it in a pan of water so i know that opens. My heaters are
warm(ish) either with or without the thermostat. Without the thermostat in and the radiator plug out the water pump seems to be pumping the water ok. So am thinking the gauge is not reading right and am not getting over 88c (ever) or somthing else is wrong . sorry for this being a bit long thanks in advance for any help its much aprectated.

what year is the 110?
sound like a conversion so my guess is that it still has the old temp gauge in, you neet to get and temp sender from the original engine and adaptor from lr series, the part numbers are in a similar thread

Hi its a 1990 yes its had a 200tdi disco put in it before i got it. how can i find out what engine was in it before so i can match the sender to the gauge thanks again.

Or just get the gauge off of a 200 defender dash, it's easier that way!