UPDATE
So I put in about 20L of cider and primed it with sugar.
I left it for 2 weeks but needed room in the fermenting fridge. Before i took it out i used a throwaway CO2 cartridge. It obviously was already gassed as the valve released the excess CO2.
I then moved it out to the cold shed.
Yesterday I thought i would give it a test. After pulling 2 pints the flow all but stopped.
I used another cartridge and left it 20min. I opened the tap and it almost exploded out.
Today I managed about another 2 pints before it stopped flowing.
This surely can be correct can it?
Any ideas?