It's definitely an overbite. Both bottom canines are behind the top ones, and one of them is slightly angled to the centre of the mouth. It is fairly pronounced, and the dentist seemed to be confident that it was highly unlikely to self correct. Just to clarify the dog is 11 weeks old currently.
For us, swapping for a new puppy is not an option, we are very attached. We will cope with the cost if it comes to it I guess I was hoping to get a small amount of assistance from the breeder is all. If it comes down to it we will deal with it on our own though.
Thanks all for the replies, I will contact the breeder and see what they have to say.