This is an appalling state of affairs resulting from the henious practice of doorstep selling.My mother was a victim of this as well but luckily in her case,I stopped the contract before it resulted in the type of problem you are now facing.You seem extremely clued up on this and some of the routes you have mentioned were ones I was not even aware of.
Our Experience: I also used my MP who contacted the company (NPower).This at least registers your complaint in the office of the Chief Executive (really their minions!!) .It gives you more chance of a successful resolution.I used energywatch who will write to the company and ask for an explanation and should negotiate on your behalf.I would try this, but I have to be honest they were less than useless in our case.I also pestered the complaints handling team who were allocated the case.Build up a friendly but firm approach with the complaint handler and use your knowledge to let them know they are dealing with someone very clued up.
Finally, Southern Electric are part of the same group of power companies who have formed the Energy Retail Organisation.They are self governing body who have developed a Code of Practice for doorstep selling.A copy of the code is held on their website.It prescribes the good practice to be followed and also gives certain remedies in the event of failure.Unfortunately,those remedies are largely left to the companies discretion,but I got our case classed as a Level 3 complaint which got us £250 compensation.It was a hard battle and I personally spoke to the Code Manager at length about it.His name is David Laird.I was not happy with the code and he did say he would take my suggestions on board.I would suggest you also contact him as he is an approachable person with overall responsibilty for the Code.
To me if a contract is misold,the company should have to honour the terms as they were presented to you.You were given assurances that they would and as agents of the company ,I believe in law they should be bound by that.
I hope Scott's intervention will also yield good results.SE have had a pretty good press here compared to other companies and they may well have picked up business because of that.I am sure Scott has pointed that out and I am sure they will be aware of the power of this forum and the resultant bad press if they do not do the right thing.
Very best of luck.