Please dont!
I have to admit that Mountain Warehouse is somewhere that I really hate, but with good reason. I think its tat, and generally overpriced tat to trap the unwary. Its not a 'proper shop', but somewhere that sells stuff constantly on 'sale' - which means its either deperate for customers, or its not a real sale (think 'Dreams' or Sports Direct'). Its the latter, and you should regard the original price as something thought up by their marketing dept. They have a 50% discount to staff (although one ex manager said he strugged to find something he'd want to buy...), which is huge. My discount is 25% (which is pretty high), and M & S is about 20%. So they are buying in that bag in at no more than £65.
Your right - its very cheap for a down bag. Which raises a question - why? The only down bags even close to that price were the ones from Alpkit (who are going to have new bags this year!), but there were very particular reasons for their prices, and were cracking value. Look at a 'proper shop' like Cotswold or Go Outdoors (their catalogues are free), and look at the cost, weight and spec of their bags, down and synthetic. Thats the standard market price for a down bag, although if you go online you might find them a bit cheaper, Ebay, etc.
Assuming that the spec of the MW one is correct, and its will be OK at minus 9 (and there is a whole thread about temp ratings elsewhere on the forum), thats about the same as a Rab Ascent 700. The Rab (which I suspect does do what it says) is a bit of a standard bag, with lots of good reviews. Its costs around £240, although you can snag it for about £210 online. So your paying about £100 extra for a bag, which you know is a bit of a benchmark. On the other hand, there is exactly one review online of this bag, last time I checked. Its on Youtube, and while its a perfectly decent review, there is no context. How does it compare with other bags, for cost weight, etc? Remember that the reviews on the MW page are largely people who have bought the bag, and said 'job done'. They seem to be 'low information consumers'. I'm sure its warm, but is it the best value?
You get what you pay for. The 'Everest'(!) weighs 1.76kg. The Rab is 1.29kg. Why? MW says there is 900kg of 'high quality' 80/20 down. The Rab also has a 80/20 mix, but only 700kg. The reason the MW weighs a lot more is because that 'high quality' down has a fill power of about 400 (as far as I can remember) Decent brands start at 600. The Rab has a 700 fill power. The Rab down is better and more efficent, hence the lower weight and less bulk. You pay for that, but its half a kilo difference, which is a fair amount.
You dont have to buy a Rab. You dont even have to buy a down bag. Cotswold have the MH Laminia 0 for £145, which slightly lighter than the MW bag, but gets you down to minus 12
http://www.cotswoldoutdoor.com/moun...0-regular-sleeping-bag-82510054?id_colour=157
.
A bloke on OM has asked about the -15 from MH -
http://www.outdoorsmagic.com/forum/gear/mountain-hardware--ultalamina-15/65280.html - Cotswold have them for £165. Thats the same temp rating as the MW bag
http://www.mountainhardwear.com/mens-ultralamina-15-regular-OU8490.html
but at only 1.18kg, way lighter.
Or Cotswold have the ME Starlight IV (minus 12) for only £89 at the moment
http://www.cotswoldoutdoor.com/mountain-equipment-starlight-iv-sleeping-bag-82510065?id_colour=98 - although its almost 2kg. The III is the same price, but a bit lighter and not quite as warm.
There are a load of sleeping bag threads on here and on Outdoor Magic, and I'm sure they will give great advice. You can spend more , or less, or about the same amount on a sleeping bag, but it has to be right, because you will be uncomfortable if you get it wrong, and you'll have wasted your money. I think that bag is cheap for a reason, and in comparison with the rest of the market, its not good value. Buy once, but right. Try Go Outdoors, Cotswold or your local retailers (your in the Peak District!), and see what they recommend. If your near Derbyshire, then drop in on Alpkit (just because they have good kit, and a clearance section), and if you search for 'Rab factory shop' on OM, you can find directions and opening times for this fabled place - which might mean a real bargain.