If, say, you want to see Rob Guest in The Phantom of the Opera, check when you book that he is scheduled to perform. But if he's ill that night, you won't be entitled to a refund. If you find yourself in this situation, complain immediately. You may be able to be reseated. If not, ask for a refund.
Some venues can arrange one on the spot.
If not, contact the ticket office the following morning, and if necessary put your complaint in writing. If neither the ticketing agent nor the promoter is forthcoming, take your case to a Disputes Tribunal. In our view, each grade of seating should be superior to those cheaper than it. Write down the reply. If you can't see as much as you were told, ask for compensation.