The best time to prune is in the autumn, but as you want to cut your trees down by half you could do it now, midwinter. You will have to be drastic, only make one large cut on each tree, the cut must be cauterized and painted with tar. Spray once a month for the next two months with Mortegg or winter wash. In the summer you will get suckers, cut them off at once, right down to the roots or base of the tree. Do not feed this year, just cultivate underneath your trees keeping it weed free.

Isn't there a danger of silver leaf if pruning now, Doctorbob, even with the precautions you suggest? Personally I'd leave it until new growth is beginning in the spring.
If you take the heads out, reducing the height, also thin out all the crossing branches and the ones going in the wrong direction.
Eventually they should crop better but you will reduce cropping for a year or two.
Dr Bob's other advice about the suckers is crucial. By heavy pruning you stimulate a lot of new spindly growth too, most of which will have to be cut out as soon as possible. Let the tree provide just a few new sturdy branches.

The Guest is not asking about pruning, as I said, prune in the Autumn. His wish is to cut the trees down by half, making one large cut cauterized with a hot iron then sealed with tar should prevent silver leaf. Thin branches can be pruned in the Autumn. Midwinter the tree is dormant, there are fruit buds on the remaining branches, more small cuts will take in silver leaf spores.