Well, I spent some time mucking around with these lights today...and then a few hours on the net, searching for some replacements. Nothing positive was achieved.
Tried the globe test, with all four of them...no noticable difference at all....battery reading was 13.8v at the terminals...and 13.1v on each socket. Seems OK to my untrained eye.
Much searching on Ebay and a couple of other places, produced some interesting results...very scarce offerings of the new variety and just a few used ones from breakers etc. Prices ranged from about 325 quid (each), up to and sometimes above 400 pounds! Very, very few of them were suitable for a LHD car. Sort of works out to be about the same price as the whole car, once I've paid for the delivery costs.
TBH, I think I'll give up driving at night.
Having said all that, I appreciate the suggestions and comments given by you all.