Thanks for the advice Phil, and sorry about the delay. I was waiting until i was out of the country before i stated on a website that i'd sold it! Ok, paranoid...
Anyway, i went to see Rene and he was mainly interested for himself, although not keen in the end as after import tax and repair he figured it would be more than the US$5,000 it would be worth as a Mongolian vehicle. But as we were driving around UB i noticed a guy in a similar Disco, although V8, and so told him what i thought was wrong and did he want it for US$1000? In about 1minute he said yes, although we were able to keep it for a few more days to get around before continuing via train, UAZ etc.
In that time i also went to the cluster of car dealers in the NE of UB, and i was offered both US$800 and $1,000 by then, so seem to be the going rate. And i'd be happy with that even if it was fine and made it to Vladivostok!
I checked at Custom's and the import tax for a >19yo Diesel of >2.5L was US$3,000 plus about $400 in GST and some other duty. About the cheapest it gets for import tax is US$2,000 for a <5yr vehicle of 2L engine.
Upon border entry the border official did enter details of car from the V5/registration document into their computer at the same time as my passport, which had me slightly concerned, but alas their systems are not linked and i exited the country to China without a problem.
I noticed that there was both a Canadian and Australian registered bikes (2) at Oasis Guest House, and after talking to the Canuck via email (both in their respective countries) i'd suggest to future travellers that bikes could be sold easily in Mongolia legal/paperwork wise, but actually finding a Mongolian that would want such bikes makes the tough bit. So either 'creativly replicating' the ownership documents or actually transferring ownership within country of registration, to sell to a foreigner like ourselves would probably be the best bet for exit.