You are going to have to follow the formal process.
Write a letter of complaint to the company and list dates, times names of people you have spoken to etc (as much as you can remember)
Vodafone will have up to 8 weeks to either resolve the problem or respond to you to give their Final Response to your complaint in which they will detail the reasons why they are not going to uphold your complaint.
Once they have issued this Final Response then this clears the way for you to approach the ombudsman if you feel that the Vodafone response was still unfair and unreasonable on your position.
When you contact the ombudsman make sure you add as much detail to the letter to them as you can, again make sure you can add as many names, dates, times as possible. check through bank statements if you need to or even if you can get old telephone records. The ombudsman will contact Vodafone and get their records of the complaint and see what action was taken ans when in comparison to your complaint.
The ombudsman will then make a decision based on what they can see, if they fall on your side then they will order recompense and insist on your credit file being adjusted, if they fall with Vodafone then you will have to settle for this. That is why it is so important to add as much detail as possible because you know that Vodafone will have dates and times against every hpione call they have received.
One thing that will be on your side is that if you threaten the ombudsman if you do not get a reasonable response from Vodafone then this can sometimes be enough for the comopany to come round to your thinking. for a case to be placed with the ombudsman means that they will have to pay the ombudsman a case fee of around £500 and just the fact of having to pay this over settling a problem worth £200-£300 may be seen as worth while to them.
Do be aware that if you do not follow the complaints procedure and just go directly to the ombudsman then the ombudsman will just refer it directly back to Vodafon as a complaint ans you will have wasted a copuple of weeks as the 8 week counter will start from this point.
Be patient and follow the procedure. I have used the ombudsman 3 times (twice against the same company) against telecommunicatiions companies and have won every time.