You can bring your case to the Pensions Ombudsman. 

You can reach them at the same address as The Pensions Advisory Service (TPAS):

Address: The Pensions Ombudsman, 6th Floor, 11 Belgrave Road, London, SW1V 1RB
Helpline Number: 020 7630 2200
Email: enquiries@pensions-ombudsman.org.uk
Web: https://www.pensions-ombudsman.org.uk/