MFU-ing anything other than a 'normal' roundtrip is a pain in the back. Did MFU an open jaw a few days ago (see relevant thread) and it was rather a clumsy and slightly chaotic process although for me it did work at the end. BA should urgently improve the part cash-part miles functionality on ba.com. Whatever it would cost them in IT improvement I'm sure would be more than saved in staff time (and indirect consequences of customers' frustration...)