I made a real rookie mistake... I followed the instructions on the BA promo and selected BA/Avios for the double dip but didn't know I also had to set the earnings choice from 'points+points' to 'points+miles'. I either read the BA promo incorrectly, or it assumed we'd have already chosen 'points+miles'.

Anyways, now sorted out before future stays, but the earlier stays can't be changed over and credited according to the call centre.

I post this in case it helps anyone else new to the game so they don't miss out!