Thanks for your reply Adrienne!
She gets on well with other dogs in most instances, she loves to meet and play with them, both with and without the other owner. She does resource guard toys and treats around other dogs, to the point of darting away from them or growling. She did once get into a fight with another dog who was trying to get a treat I was giving to Ripley. When she is tired or overstimulated (for example when we've spent too long at the park) she is more likely to become growly or fighty with other dogs. But 90% of the time she gets on well with dogs.
She gets on extremely well with people, with or without pets, loves meeting them and getting attention. She even pulls on her lead to go to people and is fine with them looming to pet her. She's never had an instance of being aggressive with other people, though she still might dart away if she's resource guarding something she's picked up off the ground.
The instances of guarding other dog owners has only happened on the lead, since they occurred whilst out on walks. The dogs were also on leads at the time, and I was also nearby. I should point out that she doesn't do this in every interaction with other owners/dogs, it's something she's started recently. She does exhibit some leash/barrier frustration on occasion.
She does startle, when there's a sudden sharp sound or if she's woken too suddenly, but she's generally not a nervous or anxious dog.
We meet the same dogs/owners on our daily walks and many different ones at the weekends when we take her to the dog park. The instances where she guarded other owners occurred with dogs/owners that we see regularly and she knows them very well. The toy/treat guarding takes place with most dogs, whether she knows them well or not. On our twice daily walks she typically will meet 2-5 people that she interacts with, some she sees often and others are complete strangers, and they are always positive experiences.
The only times she's shown signs of aggression towards other dogs are as I stated above: when guarding, when tired/over stimulated, and sometimes if the dog is smaller than her she can get pushy with them.
The only people she's ever shown aggression to have been my partner and I when we've tried to take something away that she's guarding, like a dangerous object, poisonous food, or another dog's toy.
Many thanks,
Claire