I've made a couple of orders now as the first order has been successful. It makes great sense with the Amex 241 voucher and it's also ideal for Clubcard deals as there are some good opportunities to save on some airport hotels/parking etc.
It also makes sense to purchase the Marriott points using a Nationwide credit card. If you're using an Amex card, you're effectively buying the extra BA miles you get for using the card at around 1.8p - not such a good deal considering you're buying them for less than 1p/mile in the first place!