The problem here is the Discovery temperature sender in the 200tdi engine is not matched to the Series gauge so you will not get the correct temperature reading. The Series temperature sender (560794) does not physically fit into the 200tdi thermostat housing – so in order to use the original series temp sender you need to get an adaptor that screws into the 200tdi housing (ERC8973) and the series temp sender then screws into that.
However Richard at www.glencoyne.co.uk has just discovered that the temperature senders that were fitted to the 200tdi Range Rover Classics are a direct fit but also a good match for the Series temp gauge. I have test fitted one and when the engine is fully hot the needle is just into the N mark. The part number is AMR1712 and looks the same as the Disco version but has a blue insulator.