Bad form, Mytyres, over my faulty tyre replacement

The company is hassling me to sign away my rights on a substandard tyre but I refuseLate last year I bought four tyres from Mytyres.co.uk, which is owned by the German firm Delticom AG. I noticed a problem with one of the tyres and a specialist company confirmed it had a fault. Mytyres agreed to send another and pick up the faulty one, but said I would have to sign a form agreeing to pay for the replacement if the manufacturer did not cover it. As I knew the tyre was substandard, I did not sign