hi angus,
you have the boiler supplier's certificate which saves a lot of hassle with club boiler inspectors. this will increase the value quite a lot. the remainder depends on the workmanship, but it looks a well made loco.
you can either sell it privately or by one of the well known commercial sellers. stationroadsteam have a good reputation. usual disclaimer. i personally think at least £4,000 with the commercial boiler plus paperwork for the boiler for an unfinished loco.
cheers,
julian