Thanks for the replies, so it sounds like any cyclinder would do and i just need an adaptor/gauge to match whichever filling hose the PCP has. I will phone around a few local air-gun or dive shops for prices of safety certificates.

What about the pressure of the cylinder? I guess it just means that the cylinder won't fill a PCP to a pressure greater than the cylinder itself? So a 300BAR would be more flexible on any PCP but a 230BAR would fill a PCP up to 230BAR?