Ignition puzzle

[Tony Griffiths]

There is no spark from the HT leads to the plug when held against the cylinder head or block, etc. Nothing at all. Everything is earthed and even hot wiring it to miss the ignition switch makes no difference. Both distributors work perfectly on another car - but not on this one. Crazy, eh! Of course, I've tried changing the distributor cap (tried three in fact) but it made no difference; each of the alternative caps also worked on the other car.
Plugs - I'll change those, just in case.