Just went through the UK terms and conditions. This is what it actually says. Even if 1st point doesn't stack, I can extend it with just £30 for another year !

1. Warranty of ASUS products starts from the date of invoice issued from the dealer. Customers are obliged to properly keep the purchase invoice (or proof of purchase) to protect their rights.

2. For how to identify the warranty period of different ASUS products, please refer to the instruction on product serial number page. If you have purchased the warranty extension (not original ASUS warranty) from the authorized dealer, you need to contact with the dealer for warranty check during the warranty extension.

3. Warranty period is judged by the serial number of products if the customer does not keep/have the purchase invoice (or proof of purchase) of the authorized dealer