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My Greenfield Anniversary 12 reconditioning is all but complete, finally!

All I have to do now is sort out a spark problem.......
I've fitted a new coil. All good.
Motor won't fire. Fuel supply all good, but the spark plug's not igniting it.
I can only get the spark plug to fire (out of the head) when I pull it slightly away from contact with the engine! A spark will jump from ground to plug, and the plug will fire nicely at the electrode.
If I earth the plug to the engine/frame it won't fire!

I've never come across this sort of problem before and it'll no doubt be something dumb.

Any ideas on what I should check guys? (Please.)

Thanks in advance.