At last! At the 7th attempt (plus an equally unsuccessful try by Carphone Warehouse), it finally worked (I think!) a couple of hours ago.
The usual stupid inconsistent dialogues in the dreadful Kies about emergency recovery and noting codes. The usual loosing contact with the phone during the process. It seemed to go around the process of downloading twice but this time eventually went ahead and did the upgrade.
The only difference this time (before resorting to a Samsung service centre - and their "punishment" for having the impertinence) was to remove the Sim and SD card. Did this make the difference....?
Anyway, I'm now Froyo-ized. Was it worth it? Don't know yet but it still managed to loose my provider (TalkMobile) settings...
[EDIT No, the settings have returned]