Thanks all - and I think I have my answer - 'do nothing'. It was the sale email yesterday that stupidly encouraged me to check the price again.
I understand that you pays your money and you takes your chances.
I would say that if I bought a suit in a sale that was then further discounted (within 30 days), I would be able to get the price difference back by returning it and purchasing it again from the retailer. I'm actually less bothered about the price difference as it's relatively small. It's the 'free room upgrade' offer that has appeared on my particular hotel that's annoying - and the email that BA Holidays sent to me to remind me that i've paid more than I needed to!