Based on your test results there is a problem from switched line to earth (possibly via neutral).
Since you tested with the switches off and loads removed the switched line core from the switch to the light fitting was not included in the test.
When you are testing at the ceiling rose then this is indicating a fault (with a voltage indicator?).
Leave the switches off and test IR at each light fitting from switched line to earth and one (or more) of them will show a low IR result.
If it is associated with storm conditions then I would start with outside lights and lights in the loft.
I will try this when I'm next home on Wednesday ad see what results it gives. Thanks for at least giving me another avenue to explore