I think the MoT requirements have changed very recently to include a visual check for a cat where one was originally fitted.The mot requires anything fitted to the vehicle to work and to pass legal requirements ie a smoke test which is all a diesel requires ...if there's no cat fitted the exhaust doesn't leak and it passes the smoke test then it's legal and if simply failed by someone thinking it should have a cat it can be appealed
Many many cases of this on the vw t4 forum .....i doubt there's many vw vans in the UK these day that still have a cat and are standard bhp