when trying to remove the seat frame on my 200, TDi two out of the four captive nuts decided to turn (and turn and turn) and one of them is above the fuel tank! Anyway how do you fix these captive nuts on and where do I get them?

The nuts are welded to bits of plate and the plate is pop riveted to the seat box if you look at the bolt hole you'll see a smaller rivet hole 1/2" or so away from it, if you haven't lost the original nut gubbins just re-rivet it back on. If you have lost it I dunno if you can buy new uns but it's not that difficult to knock one up.