I called them this morning, using the number Trev mentioned - sited about 10 minutes before I got through to a person, which I didn't think was bad. Then spent about 25 minutes talking to the guy, who was helpful, but not able to totally resolve the new FC number migration issue. He also had to keep going off 'to check something with someone'. Eventually, he suggested I left everything for a few minutes and then tried again and offered to call me back. We agreed on 12 noon - you won't be surprised to read that he of course didn't call back! In the meantime though, we'd sorted the problem by leading cookies....
Have to admit, I felt a bit sorry for the guy, he seemed under a lot of pressure. Might it not have made things easier if there could have been some kind of automatic migration set up? Or maybe that isn't something that can be done?