They said I intentionally broke the strap

I bought a car booster seat from Baby Boom. The car seat harness was too small to fit my daughter's arms through even though she in that weight group. The seatbelt put across the seat also didn't fit - she was too small. I took it back but they woouuldn't refund me. The next day I barely managed to get my daughter's arms through the harness strap and I pulled it tight at the bottom. The strap came loose. I took it back to shop but she wouldn't refund me, said she needed to phone supplier, Safeway, first. I showed her the consumer protection act but she didn't care. The Manageress just phoned me and said that the supplier said it could not have come loose just by pulling it, that I must have tampered with it. I see that Safeway do not respond to hellopeter. I take personal offence to this. Does anyone know what further action I could take. Is there a charge I can lay against them? Has anyone had this problem before - please, I need advice.