Hi WGT. I'm not sure exactly what type of insulating you're doing in your attic. If you're doing the ceiling ThermaWrap is an excellent option but you'd need to put it on properly with special tape etc. My friend had it installed properly in her holiday home attic by the builder who converted it and she said its insulating properties are incredible! I'd advise you get someone in the know to do it if you're not great in the DIY department. The price you were quoted does seem very competitive to me.
Where do you live? B&Q in Mahon Point in Cork are doing The Space Blanket (200mm x 4.0m) for €9.15 so the price you were quoted for that is quite good also. It'd normally be dearer in both outlets. I would strongly recommend to you, however, that you pay a few bob more and go for the thickest insulation possible. It'll be worth it in the long run and the cost difference won't be great. And don't make the mistake that I made of compressing the insulation and thinking that you'll increase the insulating properties of it. It has the opposite effect, I've been told.
You would only insulate under you converted attic floor in order to insulate the room underneath it and it would be advisable to do so if it's a possibility. You may have to take up the floor first, however, and that could be messy.
Hope all that is of benefit to you.