One CAA surveyor I spoke to said that he'd walked around an airfield and spotted several aircraft with new upholstery and hoped that the owner/operators had got modification approval (minor mod) for the change. Be aware be very aware!!!
I believe for new material you would need a Form 8 from the supplier to prove fire resistance. And, I suppose it brings into question, do upholsters need to be approved in by the CAA in someway?.