Surely GWR▸ would have known this yesterday?
Guildford to Gatwick was to run today as an isolated operation. Perhaps the same problem as Slough to Windsor and Eton where the trains wouldn't start without expert attention. 3 Units needed but if they were all in a single siding and the one at the front didn't play ...
It'll be something like that. In the end they lost the first seven trains, attributed as "due to a problem at the depot". Then they got going, for the rest of the day (so far). They don't like saying they don't know how many service will be cancelled, do they? Hence the no less embarrassing situation where cancelled trains come back to life.