Right, so an update on the Fixed.....
I had the new chain on for the DIY in the weekend and it did make a bike difference, a lot less noise. There was still clicking when I put any reasonable power through the cranks and on hills it was making all kinds of clunking and clicking noises.
On Sunday I took the cranks off and checked all the bolts on the chain ring which were all nice and tight. I had noticed on the tour that there did seem to be a gap between the Bottom bracket and the shell which I had forgotten about. Sure enough I put the BB tool in and it practically undid itself. I took the whole BB out, cleaned it, greased it and put it back in and tightened it up properly.
The result, on the ride over to Witham was an absolutely silent drive and it resolved the issue with the cracks touching the chain stay under heavier loads as well.....although an attack on the back of the drive side crank with the Dremmel helped that already!
I put the 17t on so have moved up to 73(ish) inches now, will see how that goes, might be some walking involved.