Never ever use Gungum to seal a new exhaust - this is marketed as a "silencer repair paste" It is part cement, part weetabix , as a result it is a b**ger to remove once set!

I would personally recommend Holts firegum which is an exhaust assembly sealant :- http://cgi.ebay.co.uk/HOLTS-FIREGUM-...20149001r28792