KatherineF wrote:
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

Sycamores are valuable landscape trees, they provide wonderful shade
in summer and are quite striking in winter... but they do become
immense so you need to have plenty of space for them to grow.
Sycamores don't propagate readily from seed nor from runners, are you
certain they're sycamores? They grow relatively fast but not so fast
that they'd shade a garden within your lifetime. I planted two
seedlings five years ago, they were three foot twigs, now they are
about ten feet tall but nowhere near mature size... it'll be another
20 years before I'll be able to sit in their shade, and still won't be
more than a mere shadow of their future self... I'm sure I won't be
around to see them become real trees. No one plants a sycamore for
themself, you plant them for your great grands. Unless you have a
third acre per tree then yes, move them or remove them.

Sycamores:
http://davesgarden.com/guides/pf/showimage/65316/