Mature height and spread can be up to 4 metres. You won't have to dig a hole that big unless it's already in a gigantic pot - the rootball can only be the height and width of the pot. Even so, you should dig the area over and incorporate some humus rich material (composted materials), but so long as you're in the UK, I shouldn't think ground dryness will be a problem during Autumn, because we usually get more wet weather during that time. Keep it well watered during dry spells for its first 1/2 years in the ground, till it's properly established.

Thanks very much Bamboo! I was just concerned that the ground would be hard from frost rather than too dry, but if it becomes dormant by, say, Nov/Dec, rather than Jan/Feb, then hopefully should still be okay. The pot it's in is about 2ft high x 1.5ft wide. Can I ask you please, are there any signs that an Acer is becoming potbound? I thought that if water wasn't draining well and the plant was producing dead branches underneath its top foliage, that these were signs, but I'm not sure as they are so slow growing.

Usually you see evidence of poor growth - it might look stunted or have shrivelled leaves. You might also see root material protruding from the drainage hole, or at least, if you can see just roots and nothing else through the drainage hole, its probably getting to the point where it needs to come out. If you're not sure, turn it out of its pot and have a look, replace it afterwards, even if only temporarily.
A word about planting - you never do it if the ground is frozen. A light frost at night is okay, but the best time for planting is really October, when the soil still has a bit of warmth in it, the weather isn't hot and there's a bit of rain around. For the ground to be frozen to any depth, though, temperatures would have to remain below zero C during the day, and fall past -10 C at nights for quite a period of time.

Many thanks for your advice Bamboo! It's still looking gorgeous in its pot with no roots protruding, so that gives me time.