Hi gransu
You need to seek advice from your local Welfare Rights Organisation about applying for a mandatory reconsideration and then appeal.
You need to contact the DWP and ask for a copy of the assessors report called a PA4 to see how they made their decision.
In answer to your question, an overpayment occurs from the date when a decision has been made.
Gary
Nothing on this board constitutes legal advice - always consult a professional about specific problems 
