Magpies chasing nesting songbirds

I live in Tunbridge Wells and we are overrun with magpies - I often count 15 in a single walk around the common. Blackbirds started building a nest in our garden and almost simultaneously 2 magpies started circling and landing on the lawn - almost as if they were marking a new territory. Today the magpies have started an attack of sorts - flying repeatedly into the hedge where the nest is as I think the eggs have been laid. It is really upsetting watching these aggressive predatory birds in action. We've got so few songbirds left in our area - just a whole load of magpies. Any advice? Thanks Lisa