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Just an update on this. I have finally, with some difficulty managed to work out how this all happens. Part of the problem has been that I don't get a paper statement, so don't always look at the online as closely as maybe I should.
So, when you get your monthly statement the figure that shows as your spending for the month, has all the refunds taken off it, so you've actually spent more than the figure showing. So far, so straightforward. But, if, during the current charging period, if you get a refund before your regular monthly payment from the previous month goes out, they take that amount from your payment, so you pay less than the monthly spend for that month.
Again so far so good. But because then in the next month, your monthly spend has had the refunds taken from it, it appears that you have had the refund twice. So the amount of the refund is added back into the total. Why on earth they do it that way I don't know. I have looked at the other credit cards and they don't do it that way. They deduct your refund from your spend for that month, but they don't count any of it towards the payment from before. So it's more easy to understand.
Anyway, the upshot of all this, the accounting was right. I didn't want to re-start paper statements, waste of paper, and then you have to shred it, but I am looking a bit more carefully at the ones I get online.