Using water

OK we may have some joy! A very clever dude at the petty next to my wife's work place in Jumeira Dubai said 'If the leak isnt coming from the bottom maybe its coming from the top!' A New expansion tank cap and 16 quid later and hey presto!!!! No more losing water!!!! Well chuffed, It has actually lost like the tiniest amount over the last week but nothing like before and I was told to also change the bleed screw where the three pipes meet above the rad, well happy!