I did look at that a few months ago when I was looking for pure argon for my TIG set but at £43 a year rental and £32 a refill (for Argoshield...pure argon is probably more), with a 9L bottle likely to last me about 5 years, it wasn't the cheapest option. Adams Gas is more expensive at £55 deposit and £50 a refill but even if the bottle only lasts 2 years, total cost is £105 vs £118. But yes, if you're likely to empty a bottle in less than a year (which is a LOT of welding), then that BOC offer is the cheapest.