Interim update:
I have tried contacting the church where Birgit was christened. They answer the 'phone for a period of 2 minutes every 3 months (OK, slight exaggeration

) - I'll try again on Monday, the next possibility - and they have not replied to our mails (yet).
General concensus of opinion here is that the records would probably not show Rose's address anyway, but I'm not going to give up on this one until we have a definite answer from the church.
The National Archives say that the records for German POWs in England were all handed over to the International Red Cross. I'm now putting out feelers in that direction, too...
Karen