Thank you people for your help. Yes Chrismahon, it is one of the new plastic tripod drinkers with the plastic key. I have been told that you have to let the water start running out before you tighten the top down and that should stop the overflowing so I shall give it another go. The other downside of the unit is that you cannot get the water tray out to clean it as it is permanently fixed between the legs, I must admit I'm not too impressed with it.
I have one of the other 30ltr tripod drinkers that has the bung at the bottom and if I can't get to grips with this new one then I'll get another one of those.