How tall is the Acer and how tall is the barrow? It will be the tap roots going through the barrel into the ground as Acers have feeding roots around the top. As Acers tend to need shallow but wide pots I would suggest that if the Acer is not too tall and barrow deep then you can safely detach it. The tree is deciduous so safe to move for a few months. I would add fresh compost when putting it in its new home.

Yes agree with kildermorie, i have tackeled this problem myself in the past and they have always taken ok, but i would do it sooner rather than later whilst its in its dorment state.

You might also want to do a careful selective pruning, to compensate for the broken roots, just before, or IMMEDIATELY after, digging it up. Keep the new tub off the ground on bricks, or the like, to avoid a repeat Seventeen years from now.

Another option could be to build or have built a nice brick planter round the barrel at the same hite and either fill the extra with soil and let the old barrel rot out or tease some of the old barrel of and fill in the void .

I like Noseypotter's suggestion to build a new larger planter around the old one and back-fill with appropriate compost. Just don't do anything that will expose the delicate roots in any hard frost at this time. You could also buy a larger pot to do this, bash out the base, split it approximately into two pieces, remove barrel bits, place new pot around acer, re-seal with an epoxy resin and then fill in the gap with tree, shrub, rose compost.

The only thing with that avkq is thow a cool idea you could be in the same boat in 14 years thanx anyway mate .