Spotted this on MoneSavingExpert forum and noticed it wasn't posted on here. Basically get a set of 24 sharpie pens for £6.99 when you buy a pencil case. Also I checked instore and if you have a 20% off card you can use that on the offer too, making the pens £5.60 plus the cost of the pencil case.
Sharpie 24 pack for £6.99 at WHSmith instore when you buy a pencil case. Found on MoneySavingExpert Forum