Assuming the unauthorised transaction isn't a result of your card being lost or stolen, then you shouldn’t be liable to pay any of the money that has been taken from you.
If the disputed transaction was a result of your card being lost or stolen then again, you should get back the total sum you paid.
If your card provider can demonstrate that the disputed transaction was a result of you acting fraudulently, or with gross negligence then it can refuse to refund you anything. But it's for your card provider to prove this, you don't have to prove that you haven't been negligent.
If the unauthorised transaction isn't a result of a lost or stolen card, you'll only get your money back if you've kept your security details safe.
If your card provider can show that the unauthorised transaction was a result of you not keeping your security details safe, then you could be liable for the first £50 of any disputed transaction.
Keeping your details safe means taking care of your card, bank and PIN details and not revealing them to anyone or letting anyone else use your card.
If your card provider can demonstrate that the disputed transaction was a result of you acting fraudulently or with gross negligence then it can refuse to refund you anything.
But again, if your card was reported lost or stolen before the transaction took place, then you should get a full refund.
If your card provider thinks you're responsible for the transaction or that you authorised someone else to use your card, then its up to your card provider to prove this.
In the same way, if the card provider thinks you've failed to take care of your card or bank details - or worse, that you've been fraudulent or grossly negligent - this is for them to prove.
You don't have to prove that you haven't been fraudulent or negligent its up to your provider to prove that you have.
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