Many thanks for your replies. A week has now passed and Charlie is still looking for Whisper. He's also following me everywhere - it was Whisper who always used to do that! More chatty too. But not going out so much, wanting to play loads and being an absolute nightmare with the christmas tree which until now he's always ignored. He always was the lapcat of the two - Whisper preferring to curl up next to me. My little boy can't understand why Whisper isn't back now, how can you explain it to a 4 year old? Thankfully, the two year old is blissfully ignorant, as far as I know - he and Whisper were great friends.
This year I'm sorting the garden out - its been delayed for a number of reasons, so I'm getting a tree as a memorial to Whisper - an acer in a large pot, so if we move it can come with us.