So, after trawling the lower end of the used 110 market, I've replaced the 1985 (dull thuds from chassis and unconverted 2.25 head etc.) one with a '92. First, thanks to all here. Brief resume of the story...
Looked at a 91 gas converted Disco. Recommended by the person who maintained my old Landie, but had no towbar and no conversion cert. Found a local certifier, but he had gone off roading, stepped out of vehicle and fallen off a rock, breaking leg. Also, I was amazed by the soft suspension when used in low range. Looked at various 110's on ebay and elsewhere, but they either started too high or failed to meet the reserve. It may be a buyer's market, but not if the sellers won't sell. Found one with a 300tdi where there should have been a 200 that sounded OK, but couldn't justify yet another trip to inspect, so agreed a lowish price. NFU asked for a statement that the engine was professionally installed. Seller said he knew the installer, so OK, but paper never came. Arranged for a mechanic local to seller to do inspection, but vehicle suddenly became 'unavailable'. Finally, almost in desperation, bought 'blind' off ebay, and got what seems a reasonable deal. Have had all belts changed, new oil, new oil pressure switch and alternator and it seems to be a reasonable example of a machine in need of a bit of tlc, but that seems to go and stop OK. It seems to have a softer suspension than my old one, so I assume that must have been uprated, which is why the Disco felt so soft to me.
So thanks for all the advice here and pointers to what the engine should look like etc.