the fact that you have had your dogs assessed for obedience recently stands you in good stead, thats assessment may prove useful. perhaps contact an animal behaviourist for their judgement on the matter (to be a behaviourist they must first be a quailified veterinarian so take what they say with a grain of salt)
as anouc said, admit to nothing, from what you have said it sounds as though your mastiff was reacting defensively regarding your partner having his back turned when they were approached by the rotti. as your previous experiences with this rotti have been anything but good (and this being a failing on the rotti's owners by not having the animal trained and socialised adequately) you could argue that your mastiff was assuming the worst (not to anthropomorphise or anything) in the rotti's approach an reacted to control the situation. of course ideally your mastiff would not have reacted, but dogs will almost always react to another dog that does not behave following the prescribed script of doggy language (unoffencive approach with suitable unthreatening body language, greeting, etc.) with your dogs being socialised (lets assume that they are) have they reacted in this way or similarly before to having an unfamiliar dog approach them while they are on the leash. being on the leash, while good for your case because they were restrained/under your control (for arguments sake) can provoke defencive reactions from most dogs as they know they are restrained and therefore cannot utilise the flight reaction if threatened, and therefore jump straight to the fight reaction.
ANOUC - i know you know more about me with this, please jump in here and correct me if i have said something stupid, i am only posting with what i would suggest, but i am no expert!
Thanks, shoo. I agree with everything you've said, and this is basically how we will be playing this out. I agree that it wasn't ideal for my partner to enter the park with them on lead for the reasons you mentioned, but other owners have the responsibility of controlling their dogs, as well, whether on lead or off lead. The onus is not entirely on me - in fact, while it's ideal if everyone knows not to bring their dogs through the gate on-lead, it's unlikely, so their should be MORE responsibility on those owners with dogs already off-lead who run over and harrass the new-comers.
As Kitmin said, a complaint was filed, so there were council guys on our doorstep last night to check that the dogs couldn't get out, and a regional officer showed up this morning to book an appointment with my partner to give his statement, and also check out the dogs. Luckily, when he was let in the front gate, the dogs didn't even get up from their morning nap, just gave him a few blinks and a yawn. He commented that they looked like good dogs. Which they are.
I'm sorry if anything I said yesterday was over the top or perceived as rude. I was pretty distraught, thinking of the worst possible scenario (my best mate being put down), and overreacting. There really isn't a lot of evidence to suggest that Kaiser viciously attacked the rottweiler for absolutely no reason, and that we are totally responsible. We are certainly prepared to do what we have to do - pay the vet bills (which I doubt will be more than antibiotics and a couple of stitches at the most), not take the dogs to the park anymore, continue their training - but I'm hoping that they won't have to be registered as anything other than our pets.
At the end of the day, there was a fight down the dog park. It happens. And hopefully the council realise this without thinking that every dog that gets into a fight down the park needs to be declared dangerous or menacing. Kaiser shouldn't have reacted, but the other owner should have also had control over his notoriously snappy, aggressive dog, as well. There is no reason for us to believe that Kaiser's reaction was totally unprovoked, and it is entirely unlike him to bite another dog, especially without reason. I will be putting all of this and what happened into a statement, and passing it onto a friend for legal comment before my partner's interview with the regional officer. I'll also be contacting my obedience trainers for some sort of statement, since they assessed both our dogs only weeks ago to determine whether both dogs could start training in property defense and personal protection, to which they said no, because neither dog was naturally driven, defensive, or aggressive enough to take on this kind of training effectively.
My partner will be attempting to make it home early enough to make contact with the rotti's owners over the next couple of days, and being seen to try to resolve the issue by the other regulars down the park. Hopefully, they will see reason, and that, in the context of an off-lead park, sometimes things like this happen.
If anyone has anything else to suggest PLEASE DO! Thanks all for your input so far.