Thanks - I booked with cash in the end as it was surprisingly inexpensive (and actually business class was only £1 more than economy, which perhaps reflects the offering on Air Nostrum!).
Even this wasn't easy though - the website showed two seats available at the quoted price (this on the IB flights with the best connection time), but trying to book two at once, the Air Nostrum leg reverted to economy with the 'business class not offered on this flight' message. On the flight selection page the exact same flights were actually listed twice, and booking each seat separately, one on each of the duplicate listings worked. They don't make life easy... but this could just be BA's crappy website playing up. This might also explain why I couldn't see two business class seats available together for my Amex voucher on that particular flight selection.