Oz
The main piece of paperwork that you need from your engineer is the CAA form AD202NR. However, this must be completed and signed by the engineer. You can then take this form, the test flight, Form AD968 (radio listing) and the Flight Manual status report to the local CAA regional office. Your enginner will have been sent the last two forms by the CAA.
The CAA office may renew the CofA whilst you wait if they have time; if not they will be posted it back to you. Best bet is to phone the office first.
ACX