Posted: 22/11/2013 at 13:37
How much root has grown into the soil below? I think I'd try to ease the root out of the soil.
I'm not an acer expert, but my instinct says raise the pot off the ground onto little pot feet or even a triangle of bricks to improve drainage, and wrap the pot itself in a few layers of bubble wrap.
You say the tree is doing quite well and it's obviously survived a few winters where it is. Apart from raising and lagging the pot I would leave it as is for the moment and repot (using ericaceous compost) in March.
But as I say, I'm not an acer expert - let's see what the others say