If you select all the entries then go File Export Vcard a file will be created with all the entries in it.
Then copy file to new machine and double click on the file on the new machine and it will offer to import the file into Contacts.
Thanks for the suggestion.
Unfortunately that is what I have tried to do but without success.
I just get the SBBOD and Address Book just goes into 'not responding' mode and has to be 'Force Quit'.
Strangely 2 other users on the same iMac seem to be able to export vCards OK.
All users use the same Application Address Book, so I can only assume that some of the addresses in my Address Book have become corrupted in some way.
Looks like I will just have to slog away for each individual address entry.