300tdi exhaust leak from manifold causes? easy fix?

hi there

it would seem I have an exhaust leak from the rear end of the manifold, there is black soot around that area. what could have caused this? and is it an easy fix? if its just the gasket I presume it should be straight forward as long as all the nuts aren't seized on?

I have a non stock fabricated exhaust system that is slightly smaller (diameter) than the stock one, could this potentially be the cause of the leak due to not enough fumes leaving the exhaust?

any help much appreciated

Yes, gasket failure is quite common, fairly simple to do, from memory you may need to remove the turbo oil in & out pipes to gain sufficient clearance to get the new gasket over the studs. Worth picking up the turbo oil seals at the same time as the gasket