Stolen Nectar Points

First, apologies for the choice of board. It's the closest one I could find, but Mods feel free to move it if there's a more appropriate one I overlooked.

Short Version: What are the cutomer's rights when it comes to loyalty points? What stops the scheme operator just taking them unfairly off the customer?

Long Version:Nectar recently blocked my account and sent me a new card. When I registered the new card, I saw that I had £55 fewer points than I expected. I called nectar, and they told me there'd been a £55 redemption in a Sainsburys in November 2017. In my world, £55 which I've been building up for a double points event or something is a not inconsiderable amount of money.

I've never been to the Sainsbury's where nectar told me the redemption happen, checked that I was at work at that time, and told them the redemption was fraudulent. I expected the points to be replaced. However, Nectar has refused several times, suggesting I'm mistaken or lying about the redemption being fraudulent. Nectar has made it clear that this is their final position and that there isn't a complaint process that could change their mind. They've suggested I contact the police, who will obviously not care to investigate a £55 fraud.

If this was a credit card, the next step would be to complain to an ombudsman or similar, but as it's not, I don't understand my rights and can't decide what the right next step is.

Any pointers about what I should do next, or what my rights are would be welcome.

Interestingly, I haven't lost the card. I still have the one that Nectar proactively blocked in my possession, but it strikes me that it would be rather easy to clone someone's card but taking a photo of it at the till and typing the number in. The fact that that blocked it proactively, but now won't accept that there's been fraud on the card also defies logic.

Thanks,