I got Invictus LVT flooring fitted last year in my new extension - it was supplied and fitted by a local authorised dealer.

They secured plywood boards to the chipboard sub floor and glued the LVT to the plywood.

We’ve since noticed visible lines showing through at the points the plywood boards meet (see photo)

The dealer has been inspect and has offered to remove a section of tiles to establish the cause (he said he can’t determine the cause unless he removes a section) - he has indicated that the cause could be either due to the plywood not being attached properly to the sub-floor or issues/movements in the sub-floor itself. I did ask him if the fitters would have checked the suitability of the sub-floor and he confirmed they would have. His point seems to be that the sub-floor might have moved since (e.g. due to subsidence)

He said that if it is determined that the issue was caused by their installation then he will replace it all at no cost. If, however, he deems it is due to issues with the sub-floor then he said that I would be liable for the cost of removing/replacing the sub section of tiles

It’s strikes me that it is definitely not in the dealer’s interest to replace it all and might be using the risk of ‘me being liable to pay’ to deter from taking it further. I’m also concerned that he is not ‘independent’ given its in his interests to argue the problem is in the sun-floor not the installation

I feel like I’m over a barrel. Any advice on what I can do and what my rights are? I really need a credible independent view. I’ve tried contacting the LVT supplier themselves but not sure what they can really do.
Thanks