Time to draw a line under this thread I think.
I've put it all back together. New spark plugs, new manifold gaskets, new CATS (
see this thread) new studs for the manifold flanges, and new 3/8 bolts for the manifold to cylinder head joint.
It's all running much much better. No chuffing, no misfiring, it's smoother and quieter than it's been for a while. I've done a trace with the Nanocom, and it all appears to be running and fuelling as it should.
What the problem was I don't know, but my original LR CATS look to be in pretty good shape, and none of the spark plugs was loose when I changed them, so I only assume it was a leak from between the head and the exhaust manifold on the left hand side. That's where the chuff was loudest.
I'll just have to live with the low compression for the time being, and keep my eye out for a used Turner engine to replace mine with...
The only thing is it's just failed to restart after the test run. I opened the bonnet and let it cool, and it started up first time. It's only ever done that once before, and of course I've tried hot starting it a few times more, and it start up just fine! Could this be low compression related do you think?