I am guessing with both of these :-
I would start with the relay, it sounds to me like it's on it's way out, getting slow to respond, if it's doing it everytime, even with the engine off, you may be able to hear the relay "click" when the lights come on.
A trip down to the crapy for a new one! (sounds better than shopping at Tesco)
Reversing lights could be the switch from the gearbox.... (if it's not some new fangled thing from the
Hope this helps