Originally Posted by Globaliser

Actually, if the Marriott purchase price was £107.90 including VAT, then that's actually £89.92 net + (20% x £89.92 = £17.98) = £107.90.

So 432 Avios on the net amount would work out to be about 4.8 Avios per £1, which doesn't make as much sense.

So I wonder whether the explanation for all of this is that the calculations are done manually, and that they got the Marriott one wrong in the way that you've set out.