200tdi chuffing

Right, about a month ago my exhaust manifold tried to part company, well 2 of the studs did anyway, I replace the studs and fitted a new gasket, within 10 miles it was chuffing again, I have sourced a new manifold checked it for flatness, cleaned and fitted with another new gasket, first start up its chuffing again, I've been round the engine/manifold and can't seem to feel anything or see anything blowing but it sounds like a single port on the manifold is blowing, I can't feel anything on the injectors.
Is there anything common that can cause this that isn't obvious?!

Have you checked the head for flatness?

head gasket to an oilway?
remove oil filler cap and look for fumes chuffing out.

There was no blow marks on the gasket, it had crushed a bit weird though, the breather is currently off the air box, that's not breathing excessively.
I'm gonna replace one of the studs tomorrow as I'm not happy with how it's threaded in, it seems a bit slack, so I'll stick a new one with a bit of loctite in and see where that takes me!

symptoms you describe could be manifold gasket or head gasket or valve not opening fully

What's the valve clearances on these James?

8 thou or .2mm

Head gasket right at back of engine stick your hand round between back of engine and bulk head while it's running and you will feel it, I had exact same when I bought mine thinking it was a exhaust gasket!

There's not been a loss of power or excessive smoke though?

Neither was mine drove it sweet down motorway at 70 ;-) you won't notice any real power loss surprisingly