This comes up so often.... the cylinders dont get filled for you, they get exchanged.
You should have an account with whoever supplied the cylinders, BOC or AP...?
Theres is a few cheaper schemes about.
Have you checked the sticky topic at the top of this forum section?
i know the score with acounts a ripoff sum places just used to exchainge em and still do but dont know anyone in our aeria just down near doncaster and sheffield 2 far