I had something similar at Kwik Fit folkestone a few years ago with the 968 cab. As he drove the car into the test bay I noticed that one brake light was not working. told the guy, and said I had a spare bulb and could change it in 5 minutes - but he insisted on doing the test and failing her. 5 minutes later, in their car park, I'd changed the bulb abd checked it worked, called him out to see it - he checked the other lights as well, then told me that as he was "in the system" with another vehicle he could not issue the pass cert for another hour, so I went home and had them post it to me!