i think im just going to say this outright so try not to be offended.
for the sake of £30 why try and cut corners etc, would you really want to be seen running about with a soda stream bottle hanging off your marker, do you really want a dodgy cheap bottle containing 3000psi next to your head?
just incase you dont realise how much 3000psi is ill try compare it for you,
i inflate my car tyres to 30ish psi. imagine a car tyre times 100 rammed into a small steel cylinder.
hhhmmm im starting to make no sense. i personally wouldnt want to play next to you or against you if you didnt have an approved and in date bottle and i hope any site wouldnt let you on with one not approved.