Cutting suddenly... not gradually losing power?
When it happens does the ignition warning light come on and do any of the instruments change reading?
Check whether the starter circuit is independent of that master switch. And the hazard warning lights.
Does the immobiliser work normally?
Keep your list of possibilities open...master switch, original column switch, hidden starter switch, something else...
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Do you already have the appropriate wiring diagrams? Including Andrew Revill's corrections? Please let me know if you'd like a set.
Jonathan