[pdurrant]

Let me check what's happened for me in the price matches I've done:

Book 1: VIP Price: £ 5.21, Match Price: £0.99, Refund: £. 4.32, Price: £0.89
Book 2: VIP Price: £ 9.98, Match Price: £4.39, Refund: £. 6.03, Price: £3.95
Book 3: VIP Price: £11.49, Match Price: £0.99, Refund: £10.60, Price: £0.89
Book 4: VIP Price: £ 7.01, Match Price: £3.94, Refund: £ 4.24, Price: £2.67*
Book 5: VIP Price: £ 7.01, Match Price: £3.94, Refund: £ 4.24, Price: £2.67**
Book 6: VIP Price: £. 5.21, Match Price: £0.99, refund: £. 4.90, Price: £0.31***

The first three seem to have the right refund.
The fourth & fifth I foolishly ordered two books in one transaction.
Initially, I was given a refund of £3.37 for one book, which would have given a final price of £3.64, which doesn't seem quite right to me. But I only queried it to get the price match for the second book. They increased the refund on the first, and gave the same refund on the second.
The sixth one I queried the refund which was initially £4.22 giving a final price of £0.99. I thought I should have got a refund of £4.32, which would have given a final price of £0.89. But when I queried it, they increased the refund to
£4.90

All I can conclude is that you should expect a refund of the difference +10% of the competitor's price. And if you don't get that, query it.

But I have no idea how they work out the refund when you query a refund!