Starting circuit challenge - intermittent fault SOLVED

Looking for ideas on an intermittient fault on my 1969 VF Valiant. Dart
Car will sometimes not turn over to start, at the same time no dash lights, no interior light and no headlights or tail lights.
This can happen quite quickly. Car can try to start, but turn key again and nothing. then later it will work again.
Interesting discovery is the the HORN works all the time. So power is getting to the horn but sometimes not reaching other electrical components.
I noticed in the wiring diagram that the horn is connected directly from the starter relay, so is one of the first items to get power.
Thought it might be an earth issue, but the horn works when everything else doesn't, so horn is still getting an earth.
And fuses must be OK because eventually I can get it to start again.
Anyone had this issue before or have an idea on what I can check?

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Ignition switch can do this, as can a failing connection at the amp meter.