Not sure if this is the right place to put a warning out, but I had some relatives come visit from Oz. They had exchanged at local FX shop in Melbourne and were given out of date £50 notes. We didn't realise (I can't remember the last time I had a £50 note!) and at a cafe they bought lunch with it. At St Pauls however they spotted it and told us they couldn't use it.

We swapped the currency over for them and after they left we took the notes to the Bank of England as it's not far from us. It would have taken a chunk of time out of their short stay if they had to go and do that.

Always worth reading up on the currency of the place your're going to visit to make sure you don't get given something that's not right!