I've asked for a refund on a defective kitchen worktop. But the provider is saying that the worktops were free, so there's nothing to refund...

Hello, I'm based in England, and had a large kitchen supplier design and fit a kitchen in my property. This was around April 2025. After a couple of months, the worktop started to bow upwards, and after some back and forth these were replaced in September 2025.

This was fine throughout the winter, but come March the worktop started to bow again.

I've contacted the supplier again to either re-design the worktops since they're not fit for purpose (there's no insulation between the worktop and the dishwasher, hence why it's getting damaged) or refund the cost of the worktops and we'll sort the issue ourselves.

They've since said that when our kitchen was priced up, the worktops were offered at a 100% discount so there's nothing to refund.

I was aware that the kitchen pricing was a bit suspect, in that we paid through the nose for our cabinets but all the worktops and appliances were 100% discounted. But I didn't think this would come back to bite me.

Where do I stand legally in this? Surely if they've provided inappropriate materials then it should be down to them to make it good?