Hi Vinya, thank you so much for your response.
He's no problem on his own - just when he knows Kate's around. I spoke with my vet earlier and he thinks it's because Henry is in his crate whilst Katie is out and about and he howls because he wants to play. They do get on really, really well - like they've been together all of their life, and one of the reasons why the breeder wanted her to come to us was because she knows what we're like with our pets and that Katie needed someone to play with (there used to be four of them at the breeders, two were brother and sister who were older than Katie and the'd play together, leaving Katie to play with her mum who would soon get bored, as you can imagine when you want a nice little sleep in the afternoon

and Henry is such a happy little boy he loves her.
The vet suggested leaving both out together and see how they get on. My only concern with this is that when I did this about 4 weeks ago the carpet got shredded. My vet thinks that top-dog has not been sorted out, and Henry was 'showing off' to Katie. She does take things off him, and gets up on the bed with whatever it is. I know when she's done it because of the short sharp woof he gives.