VAT reduction.

No you didn't. For starters, the reduction in VAT rate from 17.5% to

15% has not actually happened yet. Even if it had, then if the shop had kept the ex-VAT price the same, passing on only the VAT reduction to its customers, then the price would have gone down to 1150/1175 of £59.99, which is £58.71, so the most you could have saved is £1.28.

You simply had the benefit of a promotion involving a 3.3% discount.

I'm taking it straight back and demanding they take more money off me.

No sign of promotion, the HDDs were not on display, you had to ask at the till and the operator fetched one.

He actually put it down clumsily and it fell off his shelf, so I said "I'd rather not have THAT one, if you don't mind". He got another.

The price reduction was a surprise.

Suppose you were to take it back for a refund, and did so next week, when the VAT rate will be lower. When they issue the credit note (negative invoice) for the refund, one might imagine they would issue it in the amount of the original ex-VAT purchase price (£49.36) plus

15% VAT, which is £56.76, i.e. you wouldn't get your whole £58 back.

Presumably, though, sense prevails in these matters and the "tax point" for a credit note which is refund-related will be the same tax point as that of the related sales invoice, so you wouldn't lose out.

Too complicated for me, and besides, I've now backed up all my user files onto it. :)

Regarding their returns/refunds policy, I've always had instant credit to my account and no questions asked.