Does any one know if the rear doors have to be still useable, as in must open. In other words can I just block them up?
Thanks
From my own direct experience, I don't think that whether the rear doors open or not is an MOT issue, so you should be able to block them up okay.
I had a self-build Merc T1 PVC which I ran it for 20 years before selling. I had bolted the rear doors together internally and used the space behind them and the fold-away bed built there as permanent, secure storage. The doors could not be opened from the outside; you first had to enter the vehicle, access the storage space, unpadlock the doors, before they could be opened. The vehicle passed its 17 Class IV MOTs without any problem with this arrangement, even though the keys to the padlocks had never been made available to the MOT Station and the rear doors could not be opened without those padlocks having been removed first.
NOTE: I had also modified the van's nearside sliding door; it was permanently padlocked shut internally and could only be opened from the inside, again with the appropriate key - the padlocks on the rear doors and sliding door were all keyed-alike - and that key was never handed over to the MOT Tester either.
If in doubt, consult the 'MOT Inspection Manual', which can be freely accessed on the Government website
HERE. Doors are covered in Section 6.2, but would appear only to refer to Driver & Passenger doors and does not apply to doors which have been modified or any loading doors.
HTH.