Sorry to see your issue. That rust has almost certainly started because of stone impact in that area, so sadly MG aren't likely to cover it under warranty.
From the way it's rusting and spreading though, there appears to be little or no sacrificial zinc layer present, which is a bit depressing to see these days.
I do worry a little bit about longevity of these cars. The 7 year perforation warranty is a warning flag, 30 years ago, before anyone was even using galvanised steels it used to be 6 years! The norm today with the usual use of pre galvanised sheet steel is 10 or 12 years and you don't usually see any rust at all to speak of on most new cars now for at least the first 10 years.
I'm not sure how long I'm keeping mine for yet, but if I decide it's going to be longer term, it's getting all the underbody and cavities flooded with Dynax, as I'm not convinced these don't need a little extra help.
Do you have any pictures of these? Is it coming from the seal between the window and the bodywork?
I think the MG ZS saw quite a few rust problems. Rumour has it that the cavities aren't wax filled and a new thin film zirconium treatment process was used for underbody rust proofing, which is less robust than the conventional process.
I've got no idea whether the 5 uses the same processes as the ZS, but I'd assume it does.
Thin film zirconium dipping is nowhere near as good as the old hot zinc phosphating baths were. This doesn't really matter too much if the manufacturer is using quality 10+ micron zinc plated sheet steels, but if the steel is less well protected...
The change was, of course, for environmental reasons, as the old method used heavy metals and was terrible in that respect.
