Not quite sure yet what my spend was using the Nationwide Visa as it hasn't posted yet. But as Raffles noted, the extra amount it costs can be justified by using a BMI MasterCard. Alternatively it could be worthwhile if you need to put spend on the Amex card to trigger the voucher.
My thoughts were that, after these Marriott points purchases, I wouldn't need any extra BA or BMI miles for quite a while, so saving a few quid was more important than gaining the extra miles at around 1.8p