If the bulb's good and the alternator's working properly then the fault lies in the wiring. There's a lead connecting the alternator to the ignition warning light. Work out which one it is & disconnect it from the back of the alternator, turn on the ignition and touch the wire to earth; the ignition bulb should light. If it doesn't, further investigation is needed....
Tried an alternator from a van I have just bought - little unknown as the van is not running and all I have achieved is to replicate the problem on the van after refitting the alternator.
Does anyone know of a company in Devon who could check out both alternators?
Weird.... I grew up in north Devon & did my mechanic's apprenticeship there! I moved to north London & there's a company here who refurbish alternators and offer a mail-order business: