The viewpoint isn't staffed until Easter, but it's still the best place for spotting him. He never leaves the valley (Riggindale - which is right at the top end of Haweswater.) He never even comes down the valley despite multiple "eagle sightings" elsewhere in the Lakes.
He's displaying well at the moment, trying to attract a female into the valley, so you'd have a reasonable chance of seeing him if you go up. Don't go beyond the viewpoint though - it causes him too much disturbance and he needs all the help he can get. If you wait until the viewpoint is staffed, you will have a much better chance of seeing him - he has several regular perches on both sides of the valley, and the RSPB volunteers can get you on to him quite easily.
Good luck.
Mike