Thought id update this,
Managed to fix leak, well maybe fix isn't the word but its stopped for now! all i could do was as suggested clean and dry stud hole and plenty thread lock,

When i looked at another head i have laying around the stud hole goes right into a water jacket and theres a domed bit where the bottom of threaded hole sticks into it, probably a bolt that was to long has been put in and burst through,,

So i guess ill get it welded up from inside whenever head is off for a permanent fix!

In typical fashion though i fix one issue and a new one comes up, was running spot on,
But now it won't start,, it has plenty spark and fuel but when cranking it might just pop if i give it throttle? no plug leads etc have been moved, if i can get it to "pop" a few times its only the cylinder second in from the pulley thats firing (its the only one which gets a warm exhaust header)

any ideas??