I got through on the phone in the end and spoke to a fantastic agent who booked the extra seat for me.

I didn’t get a complete answer to my question. But some things I’ve learned about the process which may be useful to anyone finding this thread in future. I paid cash for an extra infant seat as no Avios seat available. If one was available some of this may differ.

The seat is booked as a cash ticket and TCP’ed to the rest of the booking. You need to speak to new bookings rather than change bookings to do this.

Advantages of going this way are that the ticket seems to get Avios; I can manage seat assignment online; I could pick Hand Baggage Only or a with baggage ticket; and I could use a BA e-voucher to pay for it.

Disadvantages are that the PNRs are split, possibly it was slightly more expensive as I think I wouldn’t have paid some of the taxes with a comfort seat, and I have lost the 1x23kg infant baggage allowance (I don’t need it on this occasion).