Local Calor Gas depot - no deposit, didn't have to buy the first bottle, no
paperwork - just strolled in & took away a 3.5 Kilo bottle of "food grade"
(i.e. for pubs etc) CO2 for 14 quid.

They have 6.5 Kilo bottles as well but those are a tad large to fit under my
tank. The 3.5 Kg bottle is just under 20 inches tall.

I.