OK, fair play to BA, and thanks for the advice. I called again, and got a very helpful guy, who sorted it all out there and then - although I was on the phone for an hour in total. I guess I've read so much stuff about how bad BA and the call centres are that I just assumed they wouldn't be able to fix it. I've ended up with two return CW flights to Rio for 210,000 and £567.80 all in.