Hi @farmandadventure, it is definitely one of the bracket fungus :

• https://www.rhs.org.uk/advice/profile?PID=98
• http://www.first-nature.com/fungi/~brackets.php

But I don't think it is the Chicken of the Woods :

• http://www.mushroomdiary.co.uk/2013/07/chicken-of-the-woods-bracket-fungus/

This page suggests some of the bracket fungus are deadly poisonous - so beware :

• http://www.ediblewildfood.com/edible-bracket-fungi.aspx

We get bracket fungi regularly on our birch and sycamore trees - but I haven't eaten one yet 😵