Heard about this yesterday in Wilko in Tooting. It involves people getting a call saying that they are due a PPI refund (or similar), but that they have to buy a certain amount of gift cards e.g. iTunes in order to get said refund. They then call again once the person has purchased the cards and get them to read out the numbers on them.
The older gentlemen on this occasion was stopped from buying several hundred pounds worth of iTunes cards, however, an older woman previously was not so lucky and lost £400. Unfortunately, there was nothing the store could do as the credit from the cards had already been spent by the scammers.
Please warn anyone you think may be vulnerable to this type of scam.