Trying to reinstall headers, and rear one has clamped up nicely front one has a large gap between head and bracket. I have tried gentle persuasion with hammer and screwdriver but doesn’t want to budge.
Do I need to loosen the rear one a little. Any ideas?

Loosen it off again, and make sure both pipes sit in the heads before tightening them a bit at a time equally.
You have removed the silencer also I take it?

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You need to make sure that NON of the exhaust system is under any strain, it has to all be in a relaxed state when fitted or you WILL break things , normally the exhaust studs. So get both headers into the ports and do up loosely, then make sure the silencer fits snuggly against ALL it's brackets. The most reliable front mount is the later dogbone type along with the Maz bracket, fitted correctly with those brackets you will have zero problems with the system . If any of it is under strain it will break .

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Thanks guys for your replies, sorry for late recognition.