From http://www.nectar.com/dynamic/estores/nectar/home It appears that you can gain nectar points through some very unsuspecting places such as the apple store, direct line, iTunes, amozon.co.uk, dell, PC World.
It appears that you use the above page link to get into the store, then you have no reference whatsoever about the fact that you will gain nectar points thoughout your shopping experience. That is the most annoying and strange bit. It would be better if you could enter your netar card number at the checkout of the relevant store, with a confirmation that you will earn nectar points.