My neighbours have big weeds or trees growing in their yard walls

Looks like a buddleiia to me . Grows anywhere. Cut it down to where it grows out of your wall and see if you can find the root entry point. You may be able to kill it with brushwood killer or try and tease it out of the crack. Is it a shared alleyway? You are supposed to be able to cut down overhanging branches, but return them to the owner!!.

yes it is a buddleia. follow Barbarak's advice.

welcome to GoY too :o)

Thanks so much

Where would i buy bushwood weedkiller??

Also would you get the ready mixed one or one you mix yourself?

I think all garden centres, plus B&Q will sell it. I think we used SBK , but there are different manufacturers eg weedol, resolva

Just thought though - you are only allowed to kill plants that are rooted on your own property so I'm assuming that you'll be doing this with your neighbour's permission?
All you are legally allowed to do is to cut off everything that overhangs your property, and this you should return to the owner -(But this can cause trouble so mention it to them before you do anything)