I've got one of these in red, its a cracking little bag. Big enough to hold a few A4 size notebooks or a Macbook pro (13 inch). Its just about the right size for carrying your laptop around.
Choose super saver for free delivery.
Also comes with a 30 year warranty, which is probably more sales gimmick than warranty, but still.
I don't know if the £5 off mastercard deal will work with this, but its worth a shot.