We eventually, 'finished' our Christmas jugsaw:
Luckily it is a Gibson puzzle and they help out in situations like this. We have to send them 5 pieces, answer a few questions, keep the box and they send us a new packet of pieces.
Each of their puzzles are individually cut so they can't send us a replacement for the missing piece.
So, when the new bag of pieces come, we shall have the pleasure of doing it again!