Thank you everyone for your suggestions and wise council (which prevented me from reaching for the big hammer).
The problem has now been resolved. I went back to the filter unit and:
Cleaned the brass spade fitting on the supply to the ignition switch, cleaned the contacts on the capacitors and connection on SW lead coil end.
Checked resistance or coil primary (Coil 1 = 2.5, Coil 2 =2.7).
Removed and Checked CB lead for continuity, cleaned contacts both ends.
Took out Distributor Base plate, thinking that the small internal lead from ignition Coil CB to Capacitor may have been shorting out against the 'U' shaped slot. Insulation was intact.
Turned over Distributor baseplate and the cause of my problem was staring me in the face - machine oil! A very fine film of oil was shorting out the insulated fitting that connects the capacitor to the leaf spring.
Cleaned up baseplate, re-assembled and with contacts open I now have 24V at the capacitor!
Re-set points gap and the old girl fired up and burst into life, big smiles all round (and charging at 28.4 volts ).
For someone with limited 'spannering ability,' the lesson I forgot and had to re-learn is that in trying to trace faults -Pay attention to the little stuff that might not initially seem important.
Thank you again.
Philip