Thanks for your replies, I'll be starting to chop them back this weekend

I don't quite understand why you say sycamores are valuable and should not be got rid of... They are weeds that grow all over where I live. I aim to kill off all the weeds in my garden in order to make it into a place where I can sit and enjoy being out doors and where I can grow plants for food. If I don't get rid of whats there, I can't do this