Well, he spent a good half hour here and has just left. He checked at the exchange before coming here, and he's going to check along the line later this morning. So far, no fault found.
Mind you, the audible crackle has disappeared this morning, as has the unstable internet connection. For the time being, all is rosy. He said it might be a wet joint; if that's the case it'll come back again eventually, but for now I'm happy. He fitted a nice new 'Faceplate 2000' while he was here so I don't need to use a separate microfilter any more.
He also remarked that had I phoned in a broadband complaint, rather than a problem with the voice service, I'd have been waiting at least a couple of weeks for him to come out. In his own words, "Virgin don't like calling us out because it costs them a couple of hundred quid, you'd probably still be arguing with them about it." A lesson worth learning there.
Needless to say, as predicted he did a full broadband health check on the line while he was here.