Just want to mention that I popped in to the Regent St store while passing through London last night. Despite not having an appointement and turning up at 8pm, I managed to get seen within about 10 minutes. The genius I saw knew is was going to be the middle screw before I told him, so it seems like it's pretty common.
He took it to the backroom and emerged about 15 minutes later. He tried progressively bigger screwdrivers (but I guess all with a 00 tip, until eventually, on the 8th chunkiest screwdriver, one that would shift it.
Anyway. Big thanks to that particular genius, and the helpful staff at the Regent St store. I've now got 2GB, so beachballs shouldn't be quite so frequent.