You are absolutely 'spot on'.
With an alternator the magnetic field is produced by the internal field coils. Although there maybe always some residual magnetism in the metal it is not like a permanent magnet so maybe not enough to create the initial alternator output because this could be very minimal or after a period of non use actually none existent.
The field coil circuit is actually initially supplied through the ignition light filament, yes the actual lamp element itself. That's the brain teaser......
So if the lamp actually fails you could get no magnetic field created in the alternator so no 12v supply output and the car would just be running on battery power until eventually that runs out and the car would stop.
To overcome this probably very rare problem that anyone would actually experience the resistors were put in parallel with the lamp filament. Should be of a similar resistance to the lamp. That way if ever the lamp actually failed the alternator field feed circuit would still be provided via the resistor.
I researched up and it is actually a modification on several forums recommended on old classic cars that have an alternator.
I don't know why Martyn's car has 2 as I see some only have one and I don't know how they have been actually physically wired into the wires. Theory is IIRC my schoolboy electronics resistors in parallel half their nominal value proportionately ie a 10ohm and a 6ohm in parallel has net effect of a single 8ohm so maybe just something in relation.
As I said nothing on schematics so I want a switchbox to poke around and test the R value figure of the resistors and how they are actually wired.
I also read that if this lamp failure did happen you can always flash a supply temporarily to the alternator field as once it is fed 12v it becomes self exciting so to to speak. It is only required for the initial creation of the alternator output.
Obviously cars fitted with an ammeter you should see it straight away too but not all classic cars were fitted with them.
Hence I can't see these resistors even if faulty would have the effect of the ignition light staying ON which is Martyn's main problem.
All the best