Agree with previous posters. A lot depends on when it was last taxed. Is there an old tax disc in the windscreen? If so, when did it expire? That will tell you whether it is on the DVLA's computer system or not. IF it went off the road before the start of their "SORN" scheme (Statutory Off Road Notification) - which was the later 1990s I think, it can stay off the road indefinitely. If it was last on the road (i.e. taxed) after the introduction of that scheme, someone should have kept re-declaring it to the DVLA each year.
Here's another link that might be useful:
http://www.dvla.gov.uk/vehicles/regoldvh.htm
Registering an "old" vehicle
In the early 1970s DVLA began to computerise records of vehicles previously maintained by local vehicle taxation offices. This process involved the registration of new vehicles and the conversion of records already in circulation.
The computer record for old vehicles was closed in 1983 and vehicles which were undergoing restoration at that time were issued with letters from DVLA. In 1990 rules were introduced which allowed vehicles to claim back their original mark, on a non-transferable basis. This means that you will not be able to transfer it or put it onto retention at a later date.
To register a vehicle which is not recorded at DVLA because it missed the 1983 deadline you will need to complete application form V765 available from DVLA Local Offices or Click here to download the form.
Further information is available in DVLA leaflet V848 - How to Register Your "Old" Vehicle.
P.M. me if you like.