Just popped over to my local TK Maxx to find they are selling these Blaze gizmo's in their clearance section for £2. These are normally £10+ elsewhere in some stores.
Basically, these things allow you to hold 3 games plugged into the device at one time and quickly select between them with the touch of a button without having to remove or insert any cartridges.
Bear in mind this was a red stickered reduced price, and although this is normally nationwide, some stores may be different, and of course stock will be different everywhere too.
This was found in TKMaxx Morfa Shopping Park, Swansea.