I was on this train on Friday. I don't know anything about what checks etc. were carried out but I think it could've been handled better by GC.
Apparently the train had an air leak, which led to it being cancelled at King's Cross just after 19:30, and everyone was told to get the 20:00 VTEC to Sunderland. At about 19:50, the GC was announced as running again. Took a couple of attempts to leave the platform (accelerate, full brakes, repeat) which I assume was the driver testing whatever the fault was. During this, the 20:00 VTEC unfortunately left, so was directly in front of us.
Just before Potters Bar, emergency stop followed by a slow crawl into the station. Train is cancelled due to overheated brakes.
Got on the next GN stopper to Peterborough where everyone was told to catch the 22:00 ex London.
I was checking RTT and saw realised the GN stopper was pulling into Stevenage 3 minutes before the 21:00 ex London, so I jumped off (along with a few others from the GC service), but the majority stayed on the GN until Peterborough.