Long sorry !
My dog, who is 4 in January, bit another dog on a group walk yesterday (with the dog walker) in an altercation over a ball. Unfortunately the other dog requires sutures
. The other dog was also on the group walk and this poor dog and my dog have been on several other walks together previously without any issues.
My dog has no form for this. He is walked off lead in the countryside and invariably greets other dogs in a friendly way - with only one or two occasions where he hasn't like another dog and has growled at them (but never been violent), so I am horrified and shocked by this turn of events.
The dog walker phoned me yesterday - the other owner is very upset (understandably) and also does not want to claim off her insurance for vet costs (again, understandably). I said that I presumed my insurance would cover it as third party liability. I have spoken to my insurance today and they wont advise me because the insurance is in my husband's name and so he has to speak to them. I can't track him down at the moment and am impatient to get to the bottom of this....
I did find an exclusion in my policy :
What we will not pay - applying to Third Party Liability
- Any costs.. .resulting from an incident which takes place when your pet is in the care of a business or professional and you are paying for their services
So it sounds like it may not be covered? Is the expectation that the Dog Walking service will pay from their insurance? 
On a separate and related note, I have made an appointment to see the vet (tomorrow) to discuss this incident and get advice concerning whether my dog needs to see a behaviourist or similar?
Any advice from any dog owners who may have insight or experience in this area?
So as not to drip feed - my dog is obsessed over balls and sticks and will try and take them off other dogs but it has always been a bit of game for him, he has not been aggressive before, just he has sometimes taken a ball and its been a hassle to get it off him and return it to its rightful owner. I now routinely put him on a lead when another dog approaches with a ball to avoid the hassle... I have previously joked that balls and sticks are like crack cocaine to him....but now that's no longer funny / a joking matter 
I am wondering if I can assume he may only be aggressive in ball / stick incidents and is otherwise 'safe'?