A man went into a shop to buy a pair of shoes for £12. He handed the shopkeeper a £20 note, and the shopkeeper did not have any change. He took the £20 note and went to the restaurant next door, where he exchanged it for twenty £1 coins. Later that morning, the owner of the restaurant came to the shopkeeper and said “this is a forged £20”. The shopkeeper apologised profusely, took back the fake note and gave the restaurant owner two good £10 notes.
Not counting the cost of the shoes, how much money did the shoe shop lose?