Has some great reviews on Amazon and has just received a 25% price drop, states ages 6+ in the description but the reviewers all seems to have kids 8/9 years old.
If you want to save another 5p Argos are doing this on click & collect for £17.19 (link in 1st post)
Create your own custom markers with the Marker Maker from Crayola. This interactive kit provides everything you need to make 16 markers a home. Whether you experiment with colours or keep things simple, this kit guides you through every step of the process. Designed for ages 8 and up, the Marker Maker includes three ink bottles, two marker boxes, two measuring tubes, mixing chart, tweezers, and enough parts to make 16 one-of-a-kind markers.