So to close this one off, it turns out that the "new bulb's" the wife bought at me request were found in a cupboard and were of the same age as the original bulbs. Not her fault, totally mine. I should have tested them first.
Thanks to everyone for the help. I started thinking.
I suspect I've found the issue. I'll report back in the morning, but I think I've missed a step in testing.... I'm possibly about to embarrass myself..
Yes I have Multimeters and enough knowledge to know how to go at it safely. I've gone and said it now....
There's been no electrical work done in a few years. The last thing done was an EV charger install 18 months ago. Installed by a pro.
There are several other 2-way switch setup's in...
I had a full rewire in 2012 during a major rebuild. All has been good since the old rubber wiring was replaced.
Recently I noticed that 2 LED bulbs in sockets linked to a 2 way switch system (It's a long corridor) had low output, flickering and sometimes didn't turn on.
I replaced the bulbs...