I had a look at it yesterday, while getting new tyres fitted. It looks like it's been rubbing against something - a whole area about 10 x 10mm is rubbed away to a depth of about 2mm. It might have been tickling the back of the wheel or suspension or something. Or someone's just caught it with a spanner while changing the pads. Getting a new one today, so will look at how it sits, to ensure it can't catch anything.
So the good news is that it hasn't perished. Now I just need to check there isn't a design fault. I doubt it though - it must have been thoroughly tested on countless Vectras.
As the car was used, I don't know its history - perhaps the same mechanic that rounded the sump plug has also been messing with the brakes. The pad wear sensor cable was also snapped on the same wheel (also being replaced today), so a heavy-handed mechanic would be my suspicion.
Will keep the old parts before I have a chat with the dealer I bought it from - not a good state to sell a car in.