Coolant leak from radiator screw/valve

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I've found my coolant leak - from a screw half way down right hand side as u look in the engine bay. Somethings not right as it's pissing out when the system gets pressurised. What size socket do I need to tighten up?

 

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anyone?

 

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Thanks for the help guys :s

Just case anyone has this problem, the bung is a 12mm hex key. Mine was cross threaded and therefore the cause of the leak. I got an m22 1.5 metal bung with built in o ring from these guys www.erentek.co.uk for free, normal cost is 70p