Seems I have a bit of blow from where the rear header connects to the rear cylinder. (I don't think there's a hole in the pipe itself). I don't think I can safely tighten those rusty old allen bolts any more. I've two shiny new ones on order, but I was wondering if adding a second aluminum fibre gasket might sort out the problem. What do you guys think?

If you think its the gasket, then replace with just a new one and new bolts, I wouldn't put an extra one in that's just giving it another extra gap through which it can blow again.

Check your tuning and make sure the rear cylinder is running a bit leaner than the front. Helps with the cooling, as you're air cooled and the front one spoils the airflow a bit.

My thinking behind adding a second gasket: if there's nothing wrong with the one already in there, (and there shouldn't as I installed the exhaust myself less than a year ago and put a new one in at the time), then perhaps a second one on top of it might allow for a more gastight seal if the problem is the pipe not sitting correctly on the rear cylinder exhaust outlet. But I'll try the new bolts first. Fingers crossed nothing gets f*cked while I gingerly take out the old ones...

Gaskets need a flat surface to work correctly, if the problem is the pipe then you would be better off grinding or filing the mating surface flat, an extra gasket could actually exaggerate the problem.

That makes sense Campaman thanks.

Job done! Had to drill two extra 8mm holes next to the exisiting ones in the metal plate holding the back end of the rear exhaust in place so that the whole thing aligns *correcty* (ya hear that Vance&Hines?? )...Oh and had to reuse the old gasket because Wemoto sent me one that was the wrong size - that's the second part they've sent me that doesn't fit - but there is now no more leakage. The new bolts worked a treat as well. I'll get the proper OEM gasket next week and fit that one to be sure. Thanks for all the advice guys!