Thankyou all for replies and kind wishes, I have told the breeder of Maggies problem she has said that she would take Maggie back and refund my money, or swap her for another pup. I coundn't do either as she is now a member of the family.
She has been seen by 3 Vets and had a scan of her heart which is already slightly enlarged and she is only 14 weeks. So surgery is the only option and she should then have a normal life.
She is going into the Royal Vet College Queen Mother Hospital on the 6th April and will have her op the following day. I will post her progress as she recovers.
Once again thank you all for your help and advise.
Karen

and Maggie