Neil posted a query on the samba about adjusting "the lay" of the pop top a few days ago, and it only just occurred to me to mention another potential issue with pop tops. Back at the rear, from curved portion to curved portion, right across the back, the pop top seal can wear on the roof and remove the paint (I think trapped grit/dust does the actual wearing away). I've seen more than a couple of vans with rust spots on the metal roof from this. So have a look at yours, if you see any wear on the paint apply some sort of tape for protection.

alistair