I put the forks in with the local bike shop to get fixed but they didn't know how to do it. Said it was a grey import so none of their tools fitted.
So I spent much of the day, trying to stop the fork internals turning by compressing the assembled forks to tighten up the bottom bolt.
In the end I used this technique, using a sash clamp and an offcut of aluminium and it didn't need a huge amount of force.
The slug of aluminium had an 11mm shallow hole drilled into it to centre the damper bar.
I made sure, when tightening the clamp there was room for the allen key to go into the bottom hole. When it was tightened up I was able to tighten the bolt into the bottom damper. Torqued to 20NM
Maybe this technique will help someone out there.