Should this fall under the builder's guarantee?

OK, this is going to be a fairly long post, but I would really appreciate any insight.

We bought our new house in April 2023. We were delighted, it looked great and the survey was great. Skip to summer 2023, and my wife dropped a dumbbell on our lovely tiled kitchen floor, revealing that the tiles had been dot and dabbed over underfloor heating, with massive voids in between! Oh no! We then realised that cracks were appearing in other places, and it became quickly evident we were going to have to replace it all.

Luckily for us, house insurance paid out on the claim, and we paid a builder £24,000 to replace almost the entire ground floor of our house in November 2023. Originally, he was going to get rid of the dot and dabbed adhesive, replace the underfloor heating, and then screed over that. However, when he started taking the tiles up, it turned out that the floor wasn't at all level, and the levels of coverage of adhesive varied pretty wildly and it wouldn't have been possible to do it that way. So instead, he used a levelling adhesive, which he laid LVT on top of. All fine, lovely new floor, no complaints.

Fast forward through winter to June 2024. The weather is getting hot, and a couple of the LVT planks by the back door started raising up, feeling like they had an air bubble under them. I got the builder back, and he glued down the planks by the door. Then, the planks just behind those ones started raising up.

So today, we took up a few rows of planks to see what was going on, and to our horror, found that the underside of some of them were soaked, and there's a crack in the screed which also feels wet to the touch. These were the planks behind the ones the builder glued down, not the ones that he actually glued down, so it's impossible to say whether the leak was already there when he last came round or it's a more recent thing, as he didn't take those planks up.

Now, the builder is saying that, as they didn't actually replace the underfloor heating system, this is nothing to do with them and isn't covered by their guarantee. Also, the guarantee is only a year, and it's been a year and a half. However, it seems to me like too much of a coincidence that he did this work, and then there was a leak so soon afterwards. And also that it surely must have happened during the year guarantee period?

However, the plumbing in this house is a joke, and I wouldn't be surprised if the people that installed the underfloor heating originally didn't do it right, so it could be nothing to do with him.

It's also hard to say exactly where the leak is coming from, as the boiler pressure hasn't been dropping, which it presumably would have if there were a leak in the underfloor heating. There is also some evidence of damp next to the back door (as shown in pictures), which I don't know if it's related or not.

I feel really overwhelmed, and I can't believe that after all the stress of construction the first time around, we might be looking at shelling out thousands more. The builder is a really nice, professional guy and I absolutely don't want to take the piss if he's right. But I also don't want to be walked all over if he's just trying to get out of doing the work! And I'm unfortunately too dumb to know the difference. Is anyone able to offer any help or advice?