Heh.
Also bear in mind that there are some cost-saving threads around, so you can probably get up to a third more material for the price you first look up in Wickes, and that it is also useful to do a heat model on the famous @JSHarris spreadsheet, and that it is worth taking a few years .. say 10 ... heating bills into account for your lifecycle cost.
eg
You can get a routine 10-20% off at Wickes itself with a Trade Accoutn and a Reloadable Cash Card, for example.
The thing about the fabric improvements is that you only get the chance to do them once, and only have to pay for them once, so some bullet-biting helps. it particularly helps when you are half under a floor at arms length with a staple gun.
F