If the vehicle is not due for taxing and has not been declared SORN, this isnt so much of a problem, but if they are caused admin hassle by not being given the correct up to date information, they will fire off fines left right and centre.

Also remember, even if the previous keeper has declared SORN, if you then take possession of the vehicle but are also keeping off the road, you will have to also declare SORN asap........ or they will fine you!