In reply to IainRUK:
> (In reply to JH74)
> [...]
>
> I just think the 'pack leader' excuse is rolled out to every problem.. this sounds more like a fear, security issue..
>
...stemming from the lack of a pack leader or someone to be in charge and tell the dog what is acceptable and what is not. It sounds like Nick is now wary of other dogs and has a fearful energy (totally understandable etc); this energy will inevitably pass onto the Jack, make her nervous (and therefore chippy etc.) herself and the vicious circle begins. If the animal is 'soothed' or similar while anxious or aggressive this will make it quickly worse.
When someone is in charge and has a good, strong and calm energy the dog will (with corrections) end up calm and less fearful and anxious (if at all). Anxiety can easily be passed on to animals and as a result they can often become a reflection of their owners.
There's a fascinating doc on BBC4 (Iplayer) at the moment about Buck Brannaman (horse Whisperer) which deals with some of these ideas.
My very remote general opinion only. Regards.