The words cheap, good quality and down don't go together. I've gone back to using synthetic sleeping bags I think down bags are overrated, they suck up moisture like nobody's business then keep you less warm than a synthetic bag anyway which will still keep you warm(er). I don't also don't like a tent full of feathers, nor stuck on my fleece so you end up looking like a chicken, synthetic is easier to care for / wash as well.
How about the shop at the Glen Nevis campsite? It is open until 9pm in July and August, 7pm otherwise. My recollection is that they have canisters, but I would email to check.
Are you having a laugh, have you seen the prices in there? £5 for a gas bottle my arse. Cotswold the other end of Fort William is cheapest for gas bottles, which is great when they're open guess not much help to the OP. That campsite reminds me of those caravanning episodes of Top Gear taking the piss. There's a reason I wild camp.
Aren't all sleeping bag ratings based on the assumption you're wearing something inside the bag? I think the Snugpak temperature ratings are exaggerated though to say the least and not as warm as they claim to be, I've got a Softie Elite 2 which is nice and tough had it for years now and it packs up small but doesn't keep you warm, not in the conditions I camp in anyway, and I know it's not a down bag but it doesn't in the slightest bit loft, they're not cheap either those bags for what they are. To be fair it's only rated at 2C comfort temp but I don't want the next one up the Elite 3 because it's too heavy.