Might be Walnut Blotch - google it for images to check. Collect up all fallen leaves, particularly in the autumn, allow none to remain on the ground, keep the area beneath the tree clean of debris. Caused by a fungus which overwinters. You can spray with a copper based fungicide such as copper oxychloride, starting in spring, but often this is difficult because the tree is so large.

At this late stage, I wouldn't bother spaying with a fungucide but collect the fallen leaves and destroy. Beware the squirrels will be watching, ready to gather in your walnuts so be on your guard.

HI "BAMBOO" AND "JIMMYTHEONE " MANY THANKS FOR YOUR COMMENTS, IM AFRAID HALF OF THE BRANCH SPREAD IS OVER A PRICKLY BUSH AREA, SO IM AFRAID QUITE A FEW LEAVES WILL ESCAPE ME . NEVER MIND I WILL HAVE TO TAKE A CHANCE. THANKS FOR THE SQUIRRELS TIP I HAD FORGOT ABOUT THEM. THANKS AGAIN, BEST WISHES "DRAKEY".