I tried this number and the voice said that this info can also be accessed from the My Virgin Media app, which I'm guessing is the same info that the Service Status web page gives? Which page always says everything is fine even if it isn't. I suppose it makes sense for this number to exist otherwise how can one check the status of the service when you have an issue with the broadband.
Anyway, I'll keep this super secret number handy.
Meanwhile I called the slightly less secret customer services number and was told to hard reset my router because of some not up to scratch performance diagnostics coming out of it. This seems to have helped, or maybe it coincided with some wider issue having been resolved, who knows.
Whatever, as long as it works I'm happy. I hope that it lasts. Thanks for your help!