It makes no difference whether you take the bike for test with or without the seat cover fitted.
If the seat cover is fitted then the tester may record on the test result 0 passsenger seats, but he is only required to record the details 'as seen at the time of test'
The only items the tester will check are that the riders seat and any rider or pillion footrests fitted at the time of test are secure.
For use on the road following test, as long as there is provision for appropriate pillion seating, (by removing the seat cover) and pillion footrests are present, then the bike, and carrying a pillion, are perfectly legal.
I have had a similar problem with a minibus that had 2 seats removed prior to test to make it fall within class 4 when it should have been class 5. As far as the mot is concerned I believe that the only reason the tester puts the number of passenger seats on the vt40 is to cover himself and confirms the state of the vehicle "At the time of test" but does not prevent anything happening afterwards.
From a discussion on the Ducatisti web site there is anecdotal evidence that if a single seat cover is attached during the MOT then the bike becomes a single seater until it is retested with the pillion seat.
I have looked through this site and others and cannot find a confirmation.
