That is what I said![]()
It's subtly different
You've said "I bought an item with a 10 year guarantee because I expected it to last 5 years" - this tells them your expectations were low and you'll probably be agreeable to a cut price settlement if they just stand their ground a bit.
You should be saying "I bought an item with a 10 year guarantee because I expected it to last 10 years and it hasn't even managed half of that" - this tells them your expectations are much higher and you're expecting a much better settlement.