We're told not to tug on the belts whilst testing, the reason being some will not retract properly afterwards if pulled too hard!
If this the only thing it fails on (even as a retest) then then VOSA need to be informed under appeal (works quickly, within a week usually) you can get an appeal form from any testing station free of charge (saves confrontation with the original test station if you're worried), you pay for another test, VOSA check it out and if your original failure is found to be unjustified then you get your test fee back, a pass certificate and the tester will be usually be sent for some retraining.
Section 5.2 of the testing manual explains the test requirements and procedures to use, nowhere is there a "tug the retracting mechanism until it locks" check!!
See for yourself, this is the official test manual online from VOSA, exactly as we use to test with....
http://www.transport...lsandguides.htm