But then I took another look at the shape of the bite mark and it all became clear.
It was obviously a Christmas card sent to Nigel by his beloved koi carp. He had been forced to leave them behind in the pond of our previous house, but they had not forgotten him.
Of course the card wasn't signed - fish can't write!
Maybe lovely Geraldine - to whom they belong now- had helped them with the address. Thanks Geraldine.