I think my problem is sorted ! Having spoken at length with the very nice man at Motad, it turns out that the silencers I had been supplied came out of the factory the wrong size. So they should be 42mm OD, but mine was 43.25mm and no wonder the gaskets got messed up. Once this was understood they replaced the silencers and new seals overnight delivery. Can't fault the after service! Will have a go at fitting tomorrow but the gasket slides on the silencer smooth.
On another note, the instructions tell me to remove the "stand stop rubber" from the old exhaust and fit it to the new one. Guess what's missing from the old exhaust. Anyone know where I can get one, its seems to fit on a welded bracket on the left hand silencer, but I can't find a picture never mind part number.
Update - Motads went on OK, used a little copper grease - make sure you have the right sized gaskets. With regard to the stand stop rubber - chanced my hand on a generic one from ebay - works great, I hadn't realised what it was a Rubber bung with a hole that just bolts on !