Thanks for your welcome messages everyone
The rescue home thought he might be crossed with an Akita but I can't see it myself. I would have loved to have seen him as a pup. I do wonder how old he was when he was turfed out onto the streets as a stray

Not that it matters any more because he has a home now!
He is a very 'mouthy' dog; he likes to chew toys and carry toys about in his mouth. He has very strong jaws and teeth and he's ripped to toys up already (they weren't exactly sturdy toys anyway) and I am slightly concerned that if left alone he might transfer this skill to bedding or furniture! We shall see!
He is clever and responsive but he is still a bit restless at home which is completely understandable seen as he hasn't even been with us a week yet. If he is asleep or sat down, the minute any of us get up or move around the house he gets up too and follows us to see what we are up to.
I think that, being a rescue dog and prior to that a stray, he is not used to having people around him all the time giving him attention. He will drift off to sleep one minute, but then he jumps up, grabs a toy and wants to play. He tends to whine whilst doing it and he rushes about picking the toy up then dropping it again, like he doesn't quite know what to do with himself.
I'm sure he'll be calmer eventually once he gets more used to us and our house.