I used to work for Autoglass and heard about this many.. MANY times. It happened for a reason you probably didn't see.. in 99% of cases it it caused by a stone hitting the back of the car.. can only be the size of a pea, but if traveling fast enough it'll go right through and shatter it without u hearing a thing sometimes.
It's not only synonymous with the Leon. It happens with every type of car every single day and is extremely common.
Unless there is a fault with your heating elements, the plug system connecting the heated rear to the cars power supply or a fault with the structure of the vehicle, you won't get a thing I'm afraid.
Insurance it'll be for you I think.. and to be honest, Seat will take over a week to get a piece of glass in for you. Better off going with who your insurance company suggest as they have 24hr access to a lot of the glass suppliers and generally keep screens for the new Leon in stock (as they're rather common).
In the last few years the majority of glass excesses has gone up to around 60-75 depending on who you're with. The claim will not affect the cost of your policy in the future, affect your no claims bonus and you will not need to declare it as a claim when you come to renew.
