You can bring your case to the Pensions Ombudsman. 

Address: The Pensions Ombudsman, 10 South Colonnade, Canary Wharf, E14 4PU.
Helpline Number: 0800 917 4487. From abroad, please dial +44 20 7630 2200.
Email: enquiries@pensions-ombudsman.org.uk
Web: https://www.pensions-ombudsman.org.uk