Just bought two 10 pack of Quavers on a multibuy deal @ Somerfield. On the back of these were TWO 2 for 1 entry to top attractions on each multipack - so that means 4 vouchers. This is sooooo much better than the Brit trip deal!!
Don't know if its on all Walkers multi's but its definitely on Quavers.
There are multiple expirys depending on where you want to go:
Alton Towers, Legoland, Thorpe Park & Chessington : 31.10.08
Madame Tussauds, Warwick Castle, The Dungeons & The Sealife Centre : 31.12.08
Only one voucher per transaction.