Thanks for the background. It fills in the story nicely. These things happen, but it would have been nice for passengers to stations to Bristol to have been given the option of hopping across to the 22:15 for a slightly earlier departure, although presumably that would have generated its own set of logistical issues...
I suspect the problem was they weren't quite sure when the 21:45 was going to depart once they became aware of additional problems with it. And as you say, so close to the departure of the 22:15 shifting everyone over to it would have caused further problems, and perhaps meant the 22:15 would have departed late itself.