looks like a sycamore leaf to me, cant say for sure as can't see the whole plant.

Looks like a Sycamore leaf with tar spot (a fungus called Rhytisma acerinum).

It looks rather like a young sycamore tree, in which case it does get those black spots, is too big for a small garden and much too big to be near a fence. A photo of the whole plant would make identification easier though as it may be something quite different.

Agree looks like a sycamore - in which case need removing.

It can be nothing but a Sycamore tree, grows very big and should be removed.

Not only does it grow big it will almost certainly drip honeydew on everything within range. Get rid of it before it gets hold!

And once it starts producing seed, you will have to pull them up every year, Derek.

Ok, Thanks everyone, I'll take the advice and dig it up, before it gets too big.

good luck!