Update:-
Lily's behaviour has improved so much since last week, and it can't be a coincidence that it has happened since having her tooth out

She was grumpy when she came home on Friday, but Saturday morning she was so happy, but very tired, and by Sunday she was her happy, bright self and we were able to take her out for a walk on her lead, no problems

I took her out for the 1st time this morning, on my own, and again she was fine.
However, the problem we still have is with her car harness, as this still seems to trigger the snarling, lunging and guarding behaviour

We were supposed to take her back to the vets tonight for her post op check, but couldn't get the harness anywhere near her -- she took it off for a chew, and guarded it whenever we went near her/it.
We were advised that we needed to use the clicker to help with getting her lead and harness on, ie, when either is on with no growling, click-treat, however, I don't know how we can practice this if we can't get her harness anywhere near her? Any suggestions will be greatly received!!