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.
I have asked the management at Bristol Water for a full breakdown of the season. As soon as it is in I will add it to my report.