I know this was on here before for £2 but this cant be done anymore and this is the cheapest they can be got.
To get this offer add something to your basket and it will ask you if you want to add some promax crisp bars. Once you have done this, remove the original item and change the quantity of the crisp bars to 2. Then on the right hand side in the voucher code box put DEC10NM, this will get you an extra £10 off.
The code DEC10NM can only be used once per account but it does give you £10 off any order.