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.
With the mild weather so far since Christmas I anticipate a great start to the fishing on Blagdon. Those lucky enough to get an opening day boat should enjoy great sport. Possibly there will be fish spread around the lake and it is hoped that floating lines and nymphs might be productive. Though it is likely that sinkers and nasties will do most damage to start the season.