Mick
As I understand it, you can pay by your debit card on Avios and accumulate your points as normal. As Olly says, you must book the holiday through Avios to get the points. I have a Lloyds Duo Credit Card, which means that you have 2 credit cards, one is American Express and the other is Mastercard. You get 50% more with American Express but a lot of companies will not accept American Express because they have to pay them very high charges, typically 10% or even more. It is too late for me this year but next year I will book through Avios using the Mastercard. The way I have worked it out on a £1000.00 is I would receive 10,000 points from Just You plus 1000 points from LloydsTSB. Word of warning though, always pay the credit card off in full when you get the bill. Not living in the UK I assume that LloydsTSB issue duo cards over there. There are different financial rules for Channel Islanders.
For anyone else reading this, who does not know what Avios Points are they are the old AIR MILES but with different rules and can be used online in many stores with online ordering.
Peter