Thanks for your message, I appreciate it is important to you.
You are correct that a dispute will first need to be made directly with them. Always try to resolve the dispute by explaining your problem and what you want done about it. They have their own internal complaints procedure you should follow.
If they refuse to do what you ask to sort out the problem, ask for a "letter of deadlock" to show you've done all you can to resolve the complaint. If they fail to respond to this final letter within a reasonable period of time (say, 14 days), you can take your complaint to the ombudsman.
You can submit a claim to the Ombudsman by using its online complaint form https://www.ombudsman-services.org/complain-now
If you have questions or would like to speak to someone you can call 0330(###) ###-####or email at***@******.***
Or you can contact them by post:
PO Box 730
The ombudsman will look at the evidence provided by both sides. They might contact you for more information, but there isn’t a hearing, as there is with a court case.
Once the ombudsman has made its decision it will write to you and the company with details of the ‘award’. If the award is in your favour (they agree with your complaint), this will include details of what the company must do to put things right.