Help please!

I brought a patio set online from a retailer back in April, and it started to rust in June (despite being covered up). I tried reaching out to the retailer, but had no luck so I raised a dispute with Tesco Bank.

I’ve sent correspondence between the retailer and I, which included pictures of the rust that clearly shouldn’t be there.

I’m now being asked for additional evidence in the form of an independent report to clarify what is wrong with the patio set!

Has anyone else had difficulties when raising a dispute like this? Given I’ve already provided photos, I feel an independent report isn’t appropriate.

TIA