Years ago I worked for a Housing Maintenance Company @ NAS Alameda.
This was Older Base Housing.
Fixtures occasionally had this problem. What I learned was sometimes as mentioned above the '"socket Connection"
would become loose. In a conventional socket (USA) the ring that you are actually screwing the bulb into is the "Neutral" and
at the the bottom of the socket it the "Hot" tab. sometimes over time the tab bends down and when used and the socket gets hot
that tab does not connect with the button on the bottom of the bulb. I would with the fixture UNPLUGED! gently pry the tab up away from the bottom of the socket, ever so gently. then when you screw bulbs in only screw in bulb gently. Plug itn the fixture and if the bulb does not function tighten ever so slightly till it lights up. Additionally I have seen folks scerew the bulb in so tightly that the small rivets that hold the neutral socket to the wire connections in back starts to get ripped off of the rivets.
It's My understanding that in the UK it is similar just at a higher voltage 230v & Neutral, and in USA 120v & neutral.
I hope this helps a lil.
Be Safe, UNPLUG before preforming any electrical work.
