Steve,
A couple of questions:
1) Does the seat belt warning light work? This is fed from the same point as the feed to the ignition ballast feed.
2) The +ve feed to the oil pressure switch also feeds the other warning lights on the instrument cluster, so do they work?
You may have more than one fault which is confusing to say the least - I had similar issues on my TR7 V8 fitted with the Pirana electronic ignition and the original Rover V8 ballast resistor box, so may be able to assist.
Cheers
Howard