You will only be able to back date a claim to a maximum of 6mths if a discresionary diecision is found in your favour. Normally they will not backdate more than three.
However if they wrongly advised you, you may be able to ask for a review of your case, you also can file an appeal of the decision.
With regard to the overpayment if you can prove you have been given incorrect advise and repaying this money will cause you severe hardship, you may not have to pay it back at all.
I would suggest you contact the CAB as they will be able to help you to make your appeal and negotiate on your behalf with the inland revenue to either write of these repayments if thats appropriate or to work out a realistic repayment shedule.
here are some links for the CAB you may find useful and also information regarding Goverment ombudsmen, If your request for a review and appeal do not resolve the issue and you still feel you have been treated unfairly it may be worth asking them to review your case, I hope this helps :-)