Given that almost all Defenders sold in Europe are diesel I guess the seller figured anyone interested would be more than familar with what a Tdi looks like under the hood. This wasn't a conversion remember, it was a stock UK farmers spec Tdi truck with the NAS bits bolted on.
It's been widely advertised in the mags for around the £16k-£18k mark so the reserve should buy it. However, since he's never actually managed to find anyone to pay his asking price, it would suggest the seller is the only one who thinks it's actually worth that much. Yeah, it's rare, but it's still a '95 Tdi 90. Standard going rate £4/5k in the UK.