Trimmed last layups and all OK. Cut away redundant part of seatback and it looks lovely. Of course I didn't have the camera on to record the success. Then proceeded to cut through the NEW redundant seatback on the other.
What an @sshole!
Was maybe bothered by that and distracted by planning how to fix it, and made a bit of a hash of the junction between the old and the stub of the new and left the foam at the join a bit thin.
Took a break, the came back and 5-min epoxy the bits back together, clamped with hot glue blocks on the front surface, reprofiled the join with pour foam, and sanded to a profile that looks nice and matches the other.
