Thanks for posting with your update as I'm still chasing my bootlid through...
They told me BMW want them to just paint it as a new bootlid costs over a grand so I said tough you gave the warranty so you pay. They also tried to be funny about not having enough services but to be honest even if they did find it sooner it would still be a replacement job so...
He's going back to BMW after I told him it wasn't good enough as you can't guarantee elimination of the rust from the inside and the warranty doesn't last much longer before I'll be on my own. Also gave some TCF (Treating Clients Fairly) warnings to him as I know others have had replacements. This will help my case now.
Fingers crossed we all get new bootlids as mine has an annoying plastic vibration now too.