Update: the known-to-be-good bulb taken from a different room and put into
the possibly-suspect fitting is still going strong after several days. The
new bulb from the possibly-suspect batch put into a known-to-be-OK light
fitting is also still burning bright.

Conclusion: several (but not all) of the bulbs in the Homebase box-of-six
had, for whatever reason, a very limited life span. There is nothing amiss
with the suspected fitting or the wiring feeding it.

Thanks to everyone for the thoughts and suggestions.

Bert