Just wondered whether the cap needs to be tightened up fully. Mine goes slack and then starts to tighten up again. Not sure whether to fully tighten or back off a bit. I have been losing a real small amount of water probably half a pint every 5k or so. Many thanks

the cap should turn freely & "click" at the point when it won't turn clockwise any more. It is then properly closed. Sometimes they get a bit stiff & some silicon spray (just a little) may help it. Or replace as they are not expensive.