You are not liable for the whole cost of the carpet, if this went to court, you (if proven to be your fault) would have to pay for the cost of the carpet minus depreciation. Ie if the carpet is 4-5yrs old the value would be approx; 50% of that amount.
It will be even less if older, which is fair when you think of it, why should you replace an old carpet with a new one. If your insurance company gets involved they will cost the same way, which will probaly be a lot less than your £500.0 excess.
Just something for you to think about, perhaps you should mention this to your customer and come to a financial agreement if all else fails.
Regards.
Tom.