It was partly way there. It new about the calendars but would not display any of the data.
I found this which did not describe my solution 100% but put me on the right track.
Go to Settings->Applications->Manage Applications->Calendar and select 'Clear data'.
This will give you a pop up box saying all the applications data willbe deleted permanently. Do not worry & select the the 'OK'.
Now go back to your calendar app and you will find all your calendars restored.
