Sainsbury's have recently started stocking SodaStream peripherals, and as it turns out I had a few empty cylinders of CO2 that needed to be exchanged.
After a bit of a palaver at the checkout trying to do the exchange because nobody knew what was going on, I learned that part of the reason the Sainsbury's employees were so confused is that the price was going through as £10 before scanning the old canisters for the refund
. So rather than return the old ones, I just bought 4 of them at £10.
The kicker is that you can return these canisters to SodaStream and they will refund you £10 per canister, rendering these essentially free at the moment. This was done at the large Sainsbury's near Vauxhall, in London. I'm not sure if it's nation wide or just this store, or whether it's an official deal or just a glitch or what, but definitely worth having a look if you use SodaStreams.