Can I ask about my plum tree please?

[Unforgettablefire]

I have a big plum tree in a not so small garden.
It's St Julien Victoria as far as I remember and what I now know is the huge kind.
If I post a photo could anyone advise where to cut it to tidy it up. I understand I can only do this in the summer so I know I have good time, I have watched countless videos and looked at hundreds of diagrams but just can't get it. I did try cutting some branches a couple of years ago but it then sent tiny shoots in the garden that keep coming back. I don't want to kill it but I might need to depending on the answer to the next question.
With this being the big kind of tree will the roots invade my neighbours garden? Or the shoots? Her garden is about 10 foot from my tree.

I was clueless about tree pruning so I paid a tree surgeon to do it and paid attention when he did. Eventually it got too big and had to be taken out. I've replaced it with some that are on dwarf rootstock so hopefully won't get too big

I would get a tree surgeon too. They can advise about whether it has/will spread and they are usually happy to give some advice on maintainance.

Thanks for the replies. I'm getting really worried now don't tree surgeons cost an arm and a leg?

Not just for one tree. Id contact a couple and see what they say as it's v dependent on location. I'm sure you could 'learn' through videos etc though so don't despair