Any time from now onwards - tree surgeons often wait till the leaves are all off before pruning these, so they can see the shape of the branches more clearly, its supposed to be easier to maintain a natural shape. It's no easy task though... but remove anything damaged, dead, or dying before you cut the rest back, and make sure the cuts are just above a leaf node, which probably will look just like a small bump on the stem in winter. The time you can't do it is once the sap has risen and up, because they bleed, so mid January up to end of July or August is when not to do it.

Thank you so much , that was a big help. Will wait till it's turned a lovely shade of red , then the leaves will fall.
Not looking forward to doing it though lol

I know what you mean, I have to trim back a large Japanese Acer in a garden I still maintain, I always go and do it in November, when it needs it, but I don't like doing it. Best to take your time, and stand back every few minutes, just to check the over view of its shape