Weak spark

I've got a 1978 Lady Dax which hasn't run for at least 15 years. I've fitted a new battery (6.4v showing), coil (with extra earth), contact points, plug cap and spark plug. However when I kick it over with just the end of the HT lead near a good earth point I just get a very weak spark . If I refit the plug cap and plug there is no spark at all. I've checked the condenser which charges ok then the voltage drops off as it should. The horn and both brake light switches work when the ignition is switched on and the neutral light works ok. Any thoughts on how to boost the spark will be much appreciated or maybe I should just call it quits and convert to 12v.

Try any of these.
Replace the HT lead.
Check plug gap.
Point gap.
Ignition timing.
Low output from flywheel magneto generator.
Remove the additional earth.