After quite a few failed attempts I managed to speak to Jenny's breeder today about getting the Kennel Club (KC) endorsements lifted so that I can register Jenny's puppies.
Di at Eastfalla had said, when we bought Jenny, that this would not be a problem as long as I had all of the required health tests done. As the results for all of these are now back and Jenny is clear, Di has said that she will write to the KC and get the endorsement lifted.
Di also suggested a breeder in Ashford as a possible source for a suitable stud dog for Jenny. I'll have to go and pay Peasblossom a visit and meet their dogs.