Co2
Might not be much use, but in my days of using Co2 you used to be able to get an adapter that fitted onto the donkey (the bit that the Co2 bottle screws into) that allowed you to use Soda Stream bottles - when I first stared out it was cheaper to buy the adapter and use Soda Stream bottles than to buy a bottle and pay for fill ups at the site - and you can get the bottles filled in any Boot's (well exchanged)..
Might not be appropriate or as cost effective nowaw-days but I thought it might be useful!
Keep the faith
Might not be much use, but in my days of using Co2 you used to be able to get an adapter that fitted onto the donkey (the bit that the Co2 bottle screws into) that allowed you to use Soda Stream bottles - when I first stared out it was cheaper to buy the adapter and use Soda Stream bottles than to buy a bottle and pay for fill ups at the site - and you can get the bottles filled in any Boot's (well exchanged)..
Might not be appropriate or as cost effective nowaw-days but I thought it might be useful!
Keep the faith