has it been grafted, can you see that the darker and lighter are from separate areas, lower /higher up the stem?

Just been to look. The darker bit is one branch half way up the tree. There are lighter branches above and below. There is only one stem/trunk. I have just noticed that the end of that branch with the dark leaves, furthest from the stem, has the lighter growth.

Hi Dorchester, most of the Japanese acers are grafted, and from the way your is growing it was possibly grafted low down. I would remove the branch as otherwise it will spoil the look of the tree.

Should I tear it off rather than cut it?

It's too big to tear Dorchester you'll need to cut off as close to the trunk as possible with loppers.

OK, thanks for that.

Beautiful tree by the way!