Who you should approach depends on what you think has gone wrong.
If you think there is a major problem that jeopardises the scheme or that it is preventing it from meeting its legal obligations then you could contact The Pensions Regulator, the body set up by Parliament to regulate occupational pensions. It would be best to contact The Pensions Advisory Service first, however, as they will be able to help you to understand the best way to raise your concerns.
Unless there is some reason not to, you should raise your concerns directly with the scheme's trustees, and with your union if you are a member. Do not forget that in almost every scheme, there are trustees chosen by the scheme's members and they should have as much interest as you in making sure the scheme is properly administered.
Schemes have actuarial advisers, part of whose job is to make sure the scheme meets legal requirements. They have a duty to report wrongdoing.