So soapy water tells me that the leaks are where the diaphragm meets the chassis - just squashed by four bolts clamping the chamber upwards.
Dismantling proved a tad tricky and unfortunately resulted in me breaking the gas chamber exhaust port brass fitting - but that's why we have lathes - make another - done

Close examination of the rubber showed that as well as the leak round the circumference there was a split in the middle that oddly didn't go all the way through.
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Rubber was 2 mm thick so I ordered some up which took ages to arrive and was actually 2.2 mm - never mind - press on and see if it works.
Well it did on compressed air on the bench but NOT on propane - more investigation required. It turns out that the genuine Zon pressure regulator that is supposed to output 1.4 bar was only putting out 0.5 bar - no wonder it's not working
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Now the pressure gauge has a bespoke needle vale attached to give a very throttled output as this regulates how frequently it goes bang.
Opening up the regulator I found it full of water

Cleaning it all up and reassembling it now will only give 0.75 bar at max adjustment - adjustment is supposed to be sealed but we are Engineers aren't we - it's only a screw!
Lots of fiddling and multiple dis-asembly and reassembly later I got it up to just over 1 bar but only by the expedient of packing the main spring with a big fat washer !
Does it work - yes by heck it does

Somewhat louder than a twelve bore shot gun, as set it cycles every two minutes with the needle valve as open as it will go. First 'shot' purges air from the pipes but after that off it pops nice and regularly