Are you MAD????
"The next day, 21 May, I called Apple. I stressed that I often leave my laptop charging overnight under the sofa on the carpet."
I cannot believe anyone would do this. Laptops require plenty of ventilation - leaving one charging overnight on a carpet is simply asking for trouble. If the lapop does not have sufficient ventilation (which in your case it doesnt) the components inside will fry - whether its an Apple, IBM or Dell laptop.
Never place your laptop on a carpet or even your duvet.
Your lucky you got any money back at all.