Hi Fraser
The repair can be carried out by laminating a veneer patch. The steps are quite straightforward – 1) Cut out the rotted area leaving a clean hole with sound structure around; 2) remove the outer and inner veneers back from the edge of the hole – i.e. outside and inside the coachroof leaving a step around the hole (shape can be square or oval): cut a central patch of agba veneer to fit the central hole and two outer agba veneer sections overlaying either side of the hole – about 50mm overlap should be okay.
Mix epoxy -I use Gurit SP106, but West System 105 /205 does the same job – either should be mixed with a filler/glue powder such as West 404 or 405 to thicken the epoxy. The final repair can be stapled and/or compressed to hold the repair in place. When finish sanded, the repair will be okay for final coat.
There is a Fairey guide on this Drawing B 12388 which is specifically for the moulded din
Best of luck