Find your local surplus shop - mine knocks them out for a fiver (which is as much as I'd pay for one). That way you can at least try them on.
To be perfectly honest, mines in the cars 'emergency kit' since about 10 minutes after I first wore it, and I cannot see why you'd want one. They are made of cotton towelling - something you will never find in a hiking shop, since it absorbs moisture like crazy. Pretty warm, but jumper like, a bit scritchy (from the towelling), but get it damp, and you'll know about it (which is why Endicotts recommends wearing it over a wicking layer. Yes, it wont melt if a cinder hits it, unlike a synthetic top, but thats about all.
Much much better to get a micro fleece - something like the Mountain Hardwear ones that Cotswold's knocks out for £25 -
http://www.cotswoldoutdoor.com/mountain-hardwear-mens-microchill-zip-top-15210198?id_colour=103 (I had had to buy another one after my wife bascially nicked mine), or ever a Craghoppers thing that they are doing for £12.50 (you'll find them at TKMaxx for about the same amount). Something like this -
http://www.cotswoldoutdoor.com/columbia-mens-fast-trek-half-zip-fleece-15220656?id_colour=107
You can wear it out and about, its comfortable and practical, and comes in regular sizes. If you want to buy a Norgie, go ahead (buy them cheap, though), but there is no way I'd walk down the street in one (that includes the Swedish version ( a bit thinner), which I got from Endicotts), and if your paying £17 for a new one, your being robbed.
Endi's is a good place to go, and I rate the French version which I got from them. Basically a glorified rugby shirt (so not exactly warm), but comfortable and practical, and doesn't look very surplus -
http://www.endicotts.co.uk/component/virtuemart/thermals-111/clothing-mid-layer-shirt-norgee-french-olive-1561-detail?Itemid=0