In the event of you having even one old tax disc showing the original registration that should be evidence enough for DVLA to grant you with that registration. Also as it is showing on the database as 'live' then again there is no reason for them to refuse you. I would ignore everything and start again, the best way to do it is to download the form (V65 I think) 'application for a replacement V5C registration document, fill it in with the required details and any that are not relevant, like axle weight, seating capacity etc, leave blank, I think there is a £25 admin fee but seeing that the tractor is still on the system you should have no problem in obtaining the document with the original registration.