Well I took the header pipes off, and bugger me sideways they didn't fit back on again. Never has this problem last couple of times.
So the centre pipe was too far over and outer too far in. Odd.
Let's see if I can post a picture. If not the concept is a long hose clamp (jubilee clip I think brits call them to pull one side in. And a block of wood as a wedge for the other.