Hello Tillylamp,
I am having the same problem with my 9 month old fell bitch, she was doing really well, but then she discovered hedghogs, rabbits, rat and now fox, now if she gets on a scent she is off there is no coming back I have to go and get her. I can't let her off even in the local park.
She does come back when she is with an older terrier. I read somewhere about giving them a break and then starting back from basics.
ATB
Vosey