I've had it running again...!

...with more success. I turned up a pair of 0.3mm thick copper gaskets to reduce the compression ratio and tested the running characteristics again but there was still some pre-ignition going on at higher revs. Back to the machine shop then, and two more copper gaskets for reducing the compression ratio even further. Now there's hardly any pre-ignition, but I'm now finding the engine stops when it gets hot and there's hardly any compression. I know the bores were a bit rough on surface finish even after honing and this is due to the cast iron being porous. I'm tempted to make two new sleeves from a different material. That way I can build in the extra thickness required to reduce the compression ratio and do away with one gasket under each cylinder head.

Here's a short video:-

Ketro KT120fsT in action