Ok, I can see whats happened here. Your original exhaust was a late model where the rear box and the center pipe conection was a sleeve type. (one slides over the other). The remus pipe and the lexmahul back box are the older but better flange fitting. The flange has a wire mesh donut stlye gasket to seal it and two sholder bolts and two springs to hold the flange together. If you go to any vauhxhall dealer they will definatly stock the bolts and gaskets becouse they often renew them when they change an exhaust, the springs they will probably have aswell. They cost bugger all but your not gonna find anywhere open till monday now.
Ive got a pair of bolts and springs in the shed, and if you were local Id offer you them until you could replace them, but noone ever posts there location in the avatar box these days?