All of the other suggestions have been covered by not only me, but three others checking for air leaks, etc.
"You may be getting sparks but are they occurring at the correct moment?"
Yes they are and if there was a problem with either ignition system, the symptoms would swap cylinders when we swapped the coils together with the plugs leads. In effect we reversed the ignition systems sending spark to each cylinder. This was further confirmed by switching off each ignition system in turn.
My next suspect is the diaphragm mechanical fuel pump, which I opened and checked that the diaphragm wasn't split. This potentially could have let fuel into the air suction line which connects to the right side cylinder inlet manifold and caused the rich mixture, evidenced by sooty plugs on that side.
I've connected the electric facet booster pump directly to the pipe supplying the carburettors and blocked the air suction line. When it stops raining tomorrow, I'll do another test run keeping fingers crossed. However I don't have great hope, because the pump diaphragm appears undamaged.