By pure chance I called at 14:35 today, as I now see recommended above, and it took me about 25mins in the queue before I could talk to someone. I explained my needs and then was on hold for about 15 mins as lots of complicated things being done but the lady came back on to say that the first thing she had done was snag the UUA seats I was after and the rest was tidying it all up and recalculating.
slightly annoyed that by changing the booking my Topcashback will no doubt be declined by worth it to sit a little further forward!