Friend has just told me she's been past Woolworths and seen this offer and she's now taken advantage of the offer too, so it definitely works.
Spend £10 or more in-store and you get a book of vouchers for 2-for-1 entry to a number of attractions, including Alton Towers.
Vouchers only work for free entry with each full paying adult but that's effectively 2for1. What's more, you only need 1 person to spend over £10 if there's a group of you. 6 of us were planning on going and she said she's got enough vouchers to cover us :)
You get 5 vouchers per booklet apparently
I'm sure they did this offer last year as well.
NB - Make sure you ask for your voucher booklet at the counter!