This is maybe just my opinion.....but a walk past BG, yeah - OK, you can probably get away with that...but purposely ringing the door, just because your a fan...that is a step too far.
MK is very private, we all know that, and BG is kind of like (in my mind) an sort of extension to Chez Knopfler and you really cant and shouldn't rock up and ring the door, just because you want to have something signed or tell MK you love his twiddly bits.
Knocking on the door of BG crossed the fan line. If you want that personal MK experience, then approaching him is fair game at a show or media event, but BG is a little too personal and fans should leave it alone and admire from a distance.
If staff come out (not because your standing outside waving banners, shouting "play Money for Nothing") and approach you then thats different - but come on...leave MK to do what he does best (albeit slowly) and keep on creating those epic albums and soundtracks