Do you mean the pilot neon is flickering ?

Yes thats what i meant.
I probably didn't explain that very well the first time around.

The boiler on neon will also remain on if something is demanding
service.

Is the spark igniting the pilot but continuing to spark and not lighting
the main burner ?

The spark doesn't appear to be igniting anything at all.
Would i expect to see at least some gas usage from the mains supply when
this is occurring? (because i don't)

The Solo 3 pcb energises the pilot valve at the same that it generates
the spark
No pilot ---- check to see if there is mains across the pilot valve
solenoid

Will do.

If the pilot valve is lit, then, if it keeps on sparking, the board is
not sensing the flame

I don't believe the pilot valve is lit.

This could be due to the pcb, or the HT lead

I'm discounting other possibilities at this point as they are less
likely and it seems to be either time or temperature dependent.

Seems likely to me also.