A man goes into a hardware shop and buys a length of rope from the shopkeeper for £3.00, handing over a £10.00 note. The shopkeeper has no change and so visits the bakery door to get some. He returns and gives the man £7.00.  After the man leaves, the baker from next door enters and says, "Hey, that was a fake note you gave me!". The hardware shopkeeper gives the baker a clean £10.00 note from the till.

The question is this: how much has the DIY shopkeeper lost?