Thank you for the information. I hope your mother in law is better..
I had already looked up medical causes and think I can rule them out for now. There are no other symptoms and her behaviour is not something I feel is 'abnormal'. It seems purely behavioural as she obviously chooses who to be aggressive to and it's more of a bluff rather than a real attack. She chooses to bully younger inexperienced dogs or older/weaker and more submissive dogs as if she feels in her old age she has gone up the pecking order. I think I am quite familiar with dog body language as I grew up with dogs.
She is most aggressive when she is with me but not so much when I am not around. She clearly is possessive and protective of me. I have tried over the years to be the 'boss' rather than her mummy, and she does listen to me, but not enough to stop her from what she wants to do instinctively.
I know I have to persevere with re-training her, but she has been a difficult dog to train purely because she is not so obsessive about food, and rather let go a treat than do something she doesn't want to do.
When she was young, exercising her and starving her then training using food worked to a certain extent, but in her old age, she is becoming less and less responsive to training using food.
Twice a day for 15 min each may be a good start so I will do that. Sorry for asking more and more questions but was wondering if you could give me a bit more advice that I haven't heard before. Thank you.