We've got a replacement!!!
No joy finding the original unfortunately, though I'll keep looking, but I found a mum on another forum that had a spare and she very kindly posted it to us.
My LO knows it isn't his original. He could tell straight away (his was a bit squashed and worn in places) but he's taken to this one quite quickly and he plays with it and cuddles it in bed. He's brightened up a lot in himself too, thank goodness!
I'm going to keep looking for another as a spare incase the worst happens again, but I've made a 'harness' for this one to attach it to my LO's clothes so hopefully it won't get dropped!
I'm a very happy mum