Duplicate payment from legit internet site

[chazza]

I ordered an item from a company on the internet (which is a legit company) and they duplicated the payment. I received the item ordered, but they charged me for two. It is only a small amount £40 (taken twice to the value of £80), and they are dragging their heels in refunding me the money.

I have sent them a (redacted) copy of my bank statement and they have agreed that yes they have made a mistake, and keep saying it will be refunded by the end of the week, which it never is.

What should my next step be? Trading standards? As they have taken money which I did not authorise them to take and now I don't think they have any intention of refunding me.