Hi, welcome to GoY, Acers are pruning group 1, so need minimal pruning, but while most prune group 1 subjects should be pruned late winter or early spring, Acers should be pruned in late autumn up to midwinter, remove any dead diseased or crossing branches, or any that spoil the symmetry of the plant, Derek.

I agree, midwinter is best because you can clearly see the tree structure and make better decisions. In my case, when all the leaves fell off, behold, a huge beehive! They tend to 'bleed' so avoid spring pruning.