Thanks for your reply but sorry, I don't understand why you suggest unplugging appliances - the RCD activates only when the MCB for downstairs lights is switched on; if that MCB is off and all other MCBs on, there is no problem.
Have I misunderstood you or have you misunderstood me?
Tuesday night - switched on fluorescent light - took several seconds to come on (usually no delay).
Wednesday night - switched on fluorescent light - did not come on.
Thursday morning - all lights & some sockets not working.
Looked at consumer unit - one of the two RCDs had activated; all...
A plug-in mains tester is showing a neutral fault on all my sockets.
These sockets have previously tested ok and I have had no electrical work done since the previous test. (I only tested today because I came across my tester whilst looking for something else.)
What could have caused this...