You chaps were absolutely correct, and I thank you for your replies!

I bought a new bulb (branded, Lucas) and before fitting it, I checked the voltage across the battery terminals - it read 12.4v. (engine not running).

I took out the bulb mount and realised that earlier I had told a porkie - whilst the first bulb had been blackened, the second bulb had gone distinctly milky. This fitted entirely with doc's statement about blackened and whitened bulbs - the second bulb must have simply gorn past its sell by date! It was, in all probability, ancient.

I sprayed the contacts with contact cleaner (used mainly for pots on the electric guitars), put the new bulb into place, and hey presto - happy bunnies. The moral of the tale is 'beware of bulbs that have been in your garage for years and years, even though they are still in their boxes.

I suppose I could simply have done what the rest of the country's motorists seem to do and left it 'til the next MoT. LOL

Thanks again .......... Waino.

There are only two things which will make bulbs fail, either excess voltage, which has been covered or vibration,

Under voltage can too. The filament can carbonise, giving a darkened or metallic look on the glass.