my acer just has not picked up this year.

You've answered it correctly already that part has died back because of the cold last winter. Cut back to live wood, or remove the dead branch at the bottom.

As you can see from photo its the whole top part of the tree. You recommend cutting whole thing down to fresh wood - including the main stem?

I don't mean cut anything that's supporting live growth. Follow the dead branch back to where it's coming from, checking whether there's any lateral growths coming from it which currently carry leaves. When you get to a point where nothing is growing from it, if that's above lateral branches, cut it there, if it's dead right to the base with NOTHING growing sideways out of it, cut it at the base and leave the leafy growth you've got.

Thanks for that. Did it last night. Looks a bit sorry for itself as the bamboo now dwarfs it. BUt i am sure it will flourish again.