I'm not an Acer expert as its too windy here, but if this were my tree and if the dead side is definitely dead (test by scratching the bark to see if its green underneath) what I would do is to cut the dead parts of, and when its dormant repot what's left setting it at an angle so that the live part is upright. You might need to support it for a while.
It looks as though it isn't planted deeply enough too, which may or may not have been a contributory factor, so try adding more compost now and put it right when you repot. There should be no roots visible so try to raise the compost level to the bottom of the trunk, even if you have to make some sort of large collar to contain it as there isn't room in the tub to raise it high enough. You can conceal the collar with a circle of bedding plants between it and the edge of the tub.

What this might be is quite a common problem with Japanese Maples. There is a bacteria that can invade any broken stems or pruning cuts to the branches during autumn and winter called Pseudomona Syringae. The upper stems die back, are often coloured black, and there may be (but not always) sunken purple blotches on the live stems.

One remedy is Bordeaux mixture, but I'm not sure if this is available any more in garden centres as the EU were considering banning it from 2014. You can still by this on eBay, I see. But it's the only successful chemical control/ remedy if PS is what you have.

The tree needs to be drenched with this stuff a few times. But start by pruning out all the dead branches first. Also keep watering of the soil to the minimum.

Sorry for delay in thanking you, still having problems with this computer. Anyway, about this bacteria and treatment if it is the cause. Ahhhh. Is it safe around children and animals, do you know? Plus when is the best time to treat?
When Acer is dormant or as soon as? I can't see the blackening on the stems etc so hoping thats not the cause, but will definately be sorting out the depth and repotting if turns out it is totally dead that side. Thanks once again.