Well as with so many questions on this forum, this one never got a response, so though I would answer it myself as it maybe of use to someone.
Bunching of coins on these escalators is common, you see all the time and can often end up with a jam. It seems it is worse on larger coins like the old penny, I guess the weight of the coin does not help. When I have had it on sixpence size coins they are often light enough just to be pushed through.
(Have simulated below picture on a fixed escalator)
Having stripped, clean, checked etc, I could not work out why this was happening. Things like spring issues or worn teeth came to mind as well as weak compression springs on the slides and grime and dirt. In my case and on another 1D escalator I had, this issue turned out to be something else. Essentially the top toothed slide should be marginally offset to the lower one so that it sits slightly behind, maybe 1mm. This allows the lower slide teeth to grip hold of the coin and the top slide back. If they are the same or the other way around the problem occurs.
There are few adjustments for this, but I did manage to adjust the slide stop on the end to compensate, which fixed the issue. Maybe of use to someone who wastes their time rebuilding again and again...have fun.