For the most part, do not bother.
The Ombudsman is an industry arranged service which is under-resourced, takes too long, lacks clear transparency.
You have no idea what evidence the FOS has received from the bank or how your complain has really been handled.
The FOS is the preferred complaints route for the banking industry.
The county court is to be preferred by consumers for its speed, openness, the quality of its awards and also because it will award 8% interest on top of any damages won by you.
The Ombudsman should normally be used to solving little technical difficulties or customer service problems which you have experienced with your bank.
Follow this link to read the damning revelations made by an Ombudsman whistleblower.
Read how the banks are contemptuous of the Ombudsman and how the service is used merely as another obstacle
Banks hold Ombudsman in contempt