Sorry about the delay, I do not generally spend much time online.
In answer to your questions
Jet is not neutered. He is about 6 months old and we plan to have him neutered at about 11 months or 1 year old.
He has done this stopping and refusing to move on walks of various lengths and there doesn't seem to be a pattern related to distance walked or time taken.
I haven't noticed any specific noises at the time. Although he is a very young dog he doesn't seem to be fazed by noises such as thunder or household appliances etc. He was not even bothered by some fireworks that were set off recently in the locality.
We did not get him until he was about 5 months old. He is a pedigree dog and had been bred as a show dog. The breeder had decided that he had some minor imperfections which would have made it impossible for him to win any trophies so we bought him. He has since had a thorough veterinary examination and he is in good health apart from a minor ear infection which we are treating. He will just be a household pet, he will not be entered for any competitions. The breeder has about 30 dogs and they are kept in a series of kennels and presumably Jet has not much experience of the outside World. He is very curious about everything around him.
I have tried to work out some kind of common factor governing the times when he has just refused to move and I can't think of any. Such things as overhanging trees, narrow pathways with high walls or hedges, different surfaces don't seem to be the answer. If you have any ideas, we would be most trateful.
Thanks for your suggestions, we will be trying them out. I am sure after some time we will be successful.