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Update.
Got the downpipe braised and replaced with new gaskets.
Went for a run and still have the noise.
So did another air test with the compressed air up the exhaust pipe to find air coming from the base of the manifold in the middle behind the exhaust pipe.
Stripped it all down to find a blown exhaust gasket in that exact place as above.
Will replace gaskets new brass nuts etc.
If I have any questions will post under a different heading.
Horse.

All back together now, went well with no more leaks.
She pulls up hills better now and surprisingly had to re set the carburettor due to a lumpy tickeover. May be something to do with the cracked down pipe or leaking manifold.
Horse.