You should notify your credit card provider of the disputed transaction as soon as you realise there's a problem.
Send your card provider any information you have that could help it work out what's happened.
For example, if you can show that you were in a completely different location from where the transaction took place, then it could help get the issue resolved if you pass that information on to your provider.
Help your card provider by sending through any information it asks for in order for it to investigate your claim.
You can use this letter template to ask your card provider for a refund if you card is stolen.
If your card provider disputes that the transaction was unauthorised, ask for your dispute to be escalated through the card provider’s internal complaints process.
If your card provider suggests that you should have taken more care to protect your card or PIN, or suggests you've acted fraudulently or that you've been grossly negligent, make the point again that this is for your provider to prove.
You don't have to prove that you haven't been negligent or fraudulent - your provider must prove that you have been.
If your card provider is still won't refund your money, you can take your claim to the FOS. The FOS has a form where you can complete and submit a claim online.
The FOS will consider all the circumstances around the transaction and may ask you to supply additional information to help it make a decision.
Once it has considered the claim, the FOS will make a decision.
The decision is binding on the card provider but not the consumer so you're free to take your claim to court if you're not satisfied with the outcome.
You should think carefully though, before you start court action and consider whether a judge is any more likely than the FOS to find in your favour.
Even in small claims, you could be liable for court fees and some other expenses if your claim is unsuccessful.