This is crap. flyclub posted in another thread that, on our advice, he achieved exactly what you are attempting- one way MFU on a return Club ticket as only available one way and then a week or two later the other leg when availability opened up.

Just call until you get someone that will agree to do the oneway MFU on the return ticket (there is an AskBA article saying just that but I can't find it atm) and then if and when availability opens for the other leg, call again until you find someone that will take the extra miles...

Why, why, WHY is it STILL such a bloody lottery with EC telesales