Symptoms were sluggush performance but no real back firing
The electric ignition was the last upgrade made
It was leads and sparks plugs
then points
At this point the back firing appeared and with the new distributor it is worse than ever

I had a friend from the classic car scene do the work for me , he is a qualified mechanic who primarily works on a Citroen DS that I have but he works on all models and is very competant
He said the same thing the valve may be stuck but he was surprised that the back firing was worse now with the new parts
Having said that he linked it back to the valve rather than the ignotion which he sees as a part of the issue but not the cause.

would correcting the valve resolve the problem or is it possible that it is a pistons problem ?