Front wipers aren't getting any water through to them despite the rear ones working. Quick search on here & then one of my first plays around under the bonnet & it looks like the non return valve is kaput (only a tiny amount of liquid comes out when activated). Popped along to my nearest Halfords to get a new one to discover they no longer do them. Having now looked online there appear to be values for both 4mm & 5mm tubing...anyone know which I need?