The recommended fluid for use in the 300tdi Discovery 1 was indeed ATF M2C 33 or LATF Dexron II D, so that would have been the fluid added to the power steering system at that time.
The recommended fluid for the Discovery 2 was listed as "Texaco Cold Climate PAS fluid 14315"
The Texaco fluid is unavailable and has been for some time, however the current LR recommended replacement for it carries a Land Rover part number, STC50519. It is available from the main dealers for about £22 a litre or on ebay for about £16 if you search on the part number.
Land Rover do say that in an emergency Dexron II can be used for topping up, but there might be the risk of it attacking the seals, as I was told by LR (Stratstones, Cardiff) themselves.