Iv got the same problem!
Just phoned my local Landy Dealer with regards this to ask about a price on a gauge!
I did have a 3.5 V8, now it has a Disco 200tdi. When i phoned up, the guy on the phone said the temp gauges are the same part number up to a certain year (Cant remember what year) and that i would just need the correct sensor for the engine! (200tdi sensor)
This was very comforting as he also said the new temp gauge would cost £135 not including VAT......... Fook rite off!!!
Bought a 200tdi Disco to 90 temp gauge from stevey parkers!
Hope this shines a bit of light!
Scott