All the books will tell you not to prune stone fruit in winter, but all the nurseries do their structural pruning in the winter anyway! Apparently, Victoria, which used to be the main plum variety grown in Britain, is paricularly susceptible to silver leaf. So up till around the 1960's, the government had a theoretical, unenforced, ban on pruning them before misdsummer. Hence the advice. But as I say, every nursery prunes in winter, and stone fruit aren't overwhelmed by silver leaf.
It's not too late to do winter pruning, but get it done ASAP. Winter pruning will produce vigorous regrowth, so use it to control the shape of the tree. If you want to lower the crown, say, cut the branches back in winter. Summer pruning doesn't lead to much regrowth, so use it to control growth. So if you want a bud to produce a fruiting spur rather than a branch, stop it in summer.
