Blagdon is a lake that responds really well to nymphs fished slowly. Diawl bachs are the "fly-of-the-lake" and in its many varieties will usually get a few fish. Buzzers also do well. As the season moves on, dry flies become more significant as a fly to put on the end. Lures such as blobs and boobies are always worth a go when the trout are not responsive to the nymphs! Long leaders are usually required (but not too long with the dry fly!) and a decent fluorocarbon is helpful - especially early season when you need to get down. Look to shallower areas and off the weedbeds later in the year.
Fish buzzers deep - using a heavy fly on the point - superglue or a fly on a heavy wire hook, and leave the flies still! Do not be tempted to retrieve as this will bring the flies too high in the water.
If we get some vloud and light winds I do expect to see some fish up in the water, unfortunately it has been wall-to-wall sunshine so far this month!