SSE stopping me switching electricity companies

Laione

I've tried on 2 occasions to switch to a cheaper tariff with other companies. SSE have blocked the transaction each time I've tried to leave. They say its because I owe them money. I pay by monthly direct debit. The first time I owed £400 on the account, the second (this time) the amount is £5.96. On no occasion have I been asked to settle either bill. Please can someone help /advise.
My electricity account used to be with them too. On my first attempt to swap I applied to switch to bulb for dual fuel. SSE allowed my electricity account to be switched but blocked my gas account which meant I lost the dual fuel offer. My electricity is now with bulb and my gas is still with SSE as they won't allow me to leave the company. Can someone please help /advise as I don't know what to do. Many thanks 

Gerry1

Write a formal letter headed COMPLAINT, ask for a deadlock letter and get a free certificate of posting.  Go to the Ombudsman upon receipt or after six weeks if they don't send one.  Their reputation is so bad that they've agreed to six weeks rather than the usual eight. Remember to come back and tell us how you get on.

Robin9

See @Laione other thread

 forums.moneysavingexpert.com/discussion/6191071/can-sse-block-me-switching-to-another-energy-company#latest