I would like to point out that VOSA does not make the rules. The law does that. Section 45 of the Road Traffic Act & Construction & use to name the main ones.
As a vehicle owner, driver etc it is down to you to ensure that it meets the minimum statutory requirements at all times while in use, not just once a year when the MOT comes around. If you obey the law, then your car would never fail the MOT!
It would appear that you have not been provided the service you paid for by at least one of the testing stations that you used so you should make a complaint to VOSA. this will put you out a little admittedly as you will have to submit your car for a re-examination by a Vehicle Examiner (better to be safe than sorry though and in your particular case should be free of charge). You will then recieve a report on your vehicle condition and a thank you letter. Then the tester(s) at fault will be dealt with accordingly. I should point out that once issued, a VT20 MOT certificate cannot be revoked and remains valid until its expiry date (the law again).
Contact VOSA on 0870 6060440
Hope this helps!