Thanks to everyone for their help.The treatment that the vet has prescribed is called Stronghold selamectin and Hazel is receiving her second and final dose tonight (one month after the first).
She is only 11 months old and has just been in season for the first time but I dont think that would cause the smell.
The majority of her coat has a normal dog smell ,it is this one bit which was bare and has now had a new growth of thick course black hair that really smells bad.
The vet did not take a scrape of her skin but we were going to wait for a while after the second dose of medicine to see if it improved.The smelly area was a bit red to begin with but now is just dry and flakey.It doesnt seem to irritate her too much.
Thanks again ,any advice is much appreciated.
Ish