As per deal over a month ago, but now cheap again (posted - http://www.hotukdeals.com/deals/blockbuster-s-pay-you-to-eat-m-m-s-/718079#comments)
Buy three of the peanut m&m's, minstrels, galaxy counters or malteasers grab bags. you get 3 for £3 instead of an average of £1.75 a bag at the moment. Inside you get a voucher code. Two codes gets you either a 6 week trial of blockbuster online, or you can download a voucher to rent a dvd for free.
SO, buy 6 bags for £6, eat the lot whilst watching 4 dvd's you rented for free :)
NB: I imagine this is available online, but I bought them instore and I believe this to be a national offer at the moment, so maybe tesco's et al will follow with similar offers :)