An update - it was a problem with my service provider. Apparantly 3G has two transmitters, one for voice and one for data. Data was working, but voice was not in my area. As the data was working, the handset showed full bars, which is rather misleading!
Not sure if I can trust 3G now, but, at least my problem is sorted.
Out of interest, Orange are now sharing transmitters with T-Mobile, Orange helpdesk sent me a link to enable my blackberry to pick up either signal depending which is the strongest.