A Plastic Bag Tax?

One idea that's been at the back of my mind for an awfully long time is to introduce a charge on plastic bags - so that they're no longer given out free. I've been frustrated by the responses that I've recieved from the big supermarkets so to take action where we can, I will be proposing that the Union shop begin to charge for plastic bags that they give out.

Unfortunately this has slipped my mind everytime I blog, which is why it's only appeared now - and I need to formalize the idea and submit it in a motion to the next Union General Meeting by Friday, so I'm open to a four-day consultation process (please tell me your thoughts via the comments link on to the post)

-Will such a charge be effective?
-How much should this charge be?
-Where should the money go?
-Any thing else that you think is relevant to this!

(Picture taken from the Brighton and Hove Green Party)