if youo can only put it in one turn and then it gets tight and starts going at a funny angle then it will be becuase the threads have not lined up correctly, If both the fill rig and tank are new and made in the uk then the threads will be the same, since there designed to go together. If however one of them is from the US then the pitch of the thread may be diffrent causing you problems. If there is a dive shop near you then i would take the fill rig there and try it in one of there bottles.