How can i retrieve all my contacts from iCloud to a new iPhone - I have no back up on a PC
If your contacts were being synced to iCloud (and if you set up a new iCloud account, I am pretty sure this is on by default), all you have to do is set up the iCloud account on the repaired phone. 

This is separate than the backup to iCloud too. If that was setup, it may also contain contacts on a restore, but they should have been backed up to iCloud with the contact sync feature that is part of iCloud.