If buyer has not paid all you can do is after two days open an Unpaid Case and close after another 4 days. That would get your selling fee refunded, but as far as payment goes there is no way you can make the buyer pay.
Legal action would be a waste of money.
To add to everything else that you have done wrong, did you really send a £40 item by 2nd Class Mail. It should have been SPECIAL DELIVERY, you are not covered by Royal Mail for loss or damage unless you use the correct method.
I would say that the buyer has taken advantage of your lack of experience and knowledge of the Postage system and has got it all for FREE.
