Hi SRH, what a great response you've had!
Just to add, Wilkinsons sell them, in the kitchen section, for about �2.99 (I know they are upwards of �12 online-stay away!)
Bodycare often have them in too, and the stationery shop The Works has them in right now too, at about �2.99. They are wonderful!
If you put a little container of water in the microwave with your bag every other time, apparently it's good for it! But it says that on the wrapper :S lol! Good luck x