It seems that however adamant the Revenue are that the £1,500 tax refund belongs to me it actually it belongs to Dyson. : (

Apparently this is because I only get a refund because of getting foreign tax credit, which I get because Dyson paid foreign tax. My contract says that Dyson can offset the cost to them using any refund due because of foreign tax credits. So I have to give them the whole £1,500. I also have to pay the revenue £27 on top of that because apparently I didn't pay enough tax on the interest on my bank account last year.

I hope I get some good karma for being honest and not just keeping the money and hoping they didn't notice…