Trying to change an avios booking via Manage My Booking (MMB) on the BA website. I cannot either change or cancel the flights and get the following error message:

"Sign in to The British Airways Club account used to make this booking before changing or cancelling"

But I am logged in using the BAC account I used to make the booking.

The error has persisted over several days. Ordinarily I would just call up and make the changes via the call centre and make sure they don't charge me any fees (beyond £35 change fee). But in this case I think the most financially advantageous option would be to cancel this booking and then rebook the flights I want to change to. My experience is that when cancelling a RFS booking via the website you get the full cash portion refunded (£130 in this case) whereas if I cancel on the phone they'll insist on taking 2 x £35 cancellation fees...

Any suggestions?