I have two Acers in large pots, last year and this year the leaves have dried and curled up, I am baffled

How long have they been in the pots they're in now? Are they potbound? Turn them out and check the rootballs - if there's not much soil and too much root, they won't be able to hold onto water because there's no soil to draw it from. Other causes could be hot sun frying the leaves (though we've not had much of that recently) and wind - if they're in a windy, exposed spot, that could frazzle the leaves too. Acers should be in a cool, shady spot and particularly dislike sunshine between about 11 and 3 in the summer.