We did this same itin two years ago. As KARFA said the open jaw redemption for Amex 241 has to be done by calling BA. Check for availability on the first leg (outbound), call BA and book it. Then check again for availability on the second leg (inbound) and ask BA to add it to the first - so everything is on one PNR. However, if your dates are available now, then just tell BA which flights you are after and they can book everything in one go for you.