Its Mountain Warehouse...
This bag has been mentioned before, although I can't find the link at the moment. I compared it with a decently reviewed down bag, and it looked very dodgy. That fact that it its temp rating is so vague is a red flag in itself. Its also quite bulky for a down bag, which makes sense when you look at the fill power of the down - look at a Robens bag from Cotswold and you will see that even that scores far better. I remember one person commented that a mate of his had bought one, and they had to get him out of it about 2am and make him do star jumps, etc, because he'd gone hypo.
You get what you pay for, and remember that Mountain Warehouse staff get a 50% discount on all stuff - so that bag is £65 at most without markup. If you want a decent down bag for a relatively low price, then, like Copperhead, I'd recommend looking at Alpkit - the Skyhighs
https://www.alpkit.com/sleeping-bags are the ones I've got my eye on. I hear good things about Mountain Hardware for synthetic, but I have to admit that when I tried one yesterday at Cotswolds, I found the foot pretty tight - but thats a personal thing.
Avoid pretty much anything by Mountain Warehouse - cheapish stuff with high markup. If you look at the reviews of that bag on their website, think just how many people actually know anything about sleeping bags. There are loads of sleeping bag threads on this site, and on Outdoor Magic, and they all seem to be along the lines of get the best bag you can for your money. This wouldn't be it.