To use the push fittings you must ½ inch OD tubing (in other words 3/8 ID from the hardware store vary thin walls. That seal on the outside of the tube so metric tubes would likely leak. You are in the UK?) If you want ½ ID tube buy the adapters. Seals on the ID should work with clear flex or local tubing
http://www.cooltechnica.com/Merchant...egory_Code=TAF
Same ID as all ½ hose barbs?