To clarify when you say scour , I'm assume you mean you look at the discs and they is a semi deep scratch in the disc. You mention a sound when it cold, if you mean when u press the brake you hear a type of squeak , very mild fog horn sound. This is very common in Corsa and apart from replacing all braking components, it's just a matter of lump it. (Mine does this). The issue is , despite you saying your mechanic is pretty good, with brakes unless you do it yourself, there.maybe a very minor thing that gets missed (with no malice, just garage's have time constraints). E.g Does he wire wheel the slider pins? Does he regrease the slider pins or just check they have movement? Does he change the brake accessories aka shims. These are all things I do for my customers but it does come at added cost purely because you need to buy the accessories, but it does add time. These things could be contributing to the issue. If you ask him he will mostly likely says he does all this but I very much doubt it. I have a very small niche of customers so have the time to take care of them all at 100%. Garages just don't have this luxury. The scouring is something that you need all these things to be done and know it's been done otherwise people will always says what I have said. The strut issue won't cause an issues with scoring on the discs.No heat at centre of both front wheels.
This has been noticable from almost new,car 7+years old now.I had thought this might be cured after having the discs an pads replaced but still the same.My mechanic is pretty good and would have cleaned and greased when replacing them.
I have read that some have had problems with the brake shield catching on the discs on the Corsa causing noise,but would this only happen when cold if that is the cause.
the car also gets an mot advisory ever year fot the Macpherson Struts,but passes.Would a bit of movement there cause the brakes to scour?