The item had cost £16 but at the Customer Services desk they wanted to refund just £8 because the item had been reduced after my purchase.
I understood their position but didn't think they hang on to £8 of my money for no benefit to me.
I've been in this situation before in Sainsbury and IKEA. This is how to handle it.
Tell them the credit card number you used to pay for it and roughly the date of purchase. Ask that they trawl through their transaction records to find out how much was actually paid.
When I've done this the record has been found and I've received a full refund.
Clearly it's easier to keep a receipt, but if you lose the receipt, remember that they can check their records in your behalf.