This morning Bernie took it for a half hour drive and took it back to be checked. It flew through the COF this time and passed with flying colours. Yay!
Although we had been told we needed to warm the brakes up prior to a test, we weren't aware that it meant driving for that amount of time. Bernie had taken it for a 10 minute warm up before the initial inspection - obviously that wasn't long enough. So that's another lesson learnt.