Just wondering if anyone knows what might happen if a dog leaves a competition ring and attacks another dog? Would the dog or owner be banned? Fined.? What sort of investigation might be done?

Depends and any complaints incidents are reported on by the society that hold the competition.

I know of one in agility where said dog went out of ring and was involved in altercation with another.
Complaints made and went off to relevant KC committee which ruled on the incident.

Yes depending on what occurs and level of the complaint fines can be put in place, dogs can be either completely banned
or their card marked and if another incident happens then they get banned.
I think in the case I mentioned as there was no physical damage to dog or handler the dog who ran out of ring had it's card marked
and so did the handler so if there is another incident they may get further action against them bans etc can't remember if they were fined.

I know both sides had eye witness statements that the organising secretary took on the day etc.

Normally the results after the relevant KC committee has sat are published in the KC Gazette.

thanks. very helpful

As has already been said a complaing would have to be given on the day.  If it wasn't then don't expect anyting to happen with the KC.

Think the KC decide on the action they take depending on the facts presented to them. For example a dog running from an agility ring and attacking another dog is one thing an exhibitor exercising a dog off lead at a show would be in far worse trouble because the dog at the agility had a right to be off lead that at a show does not. A lot will depend on if the persons concerned wish to take action or not, penalties can range from a warning through to the dog and it's progeny being banded.