We have an outside tap in the side passage that runs off the mains

May I just check that this means the kitchen tap is (as it shd be) fed
direct from the supply pipe and not from the attic tank?

Would hugely appreciate some advice or even answers.......

This is more a matter of elimination. Is it possible that the kitchen
tap is siphoning from the attic tank when other taps are used? In other
words, is the supply to the attic tank under water when the attic tank
is full? (I assume there is a ball-cock there which may just possibly
now be letting the level get too high.)

--
R