Finally sorted this issue, in case anyone else needs to book a lap infant you need to call BA and transfer to the executive club desk where they can issue a new lap infant ticket under a separate reservation. They only accept Avios points and it's charged 10% of the BA award. In our case from Zagreb to Boston it was 9500 Avios and $100 in taxes. Happy to report it was not thousands of dollars but make sure you have Avios for the lap infant ticket.