have you had the gauge working already since you changed the engine,
i had this issue 2 weeks ago, the info i had said that the temp sender was the same between disco and defender 300tdi engines, they are not , disco sender has a green end , defender one has a black end, theres lots of info on the web about gauges not working etc.
i ended up buying a new 52mm gauge and sender kit from ebay and replaced both, it reads actual temp not just an indication
300 should run at 95c
ebay no 380378149462