none of it's incorrect – the age of the box in the OP has already been agreed as borderline for reasonable duration. they would likely argue they're not compelled to do anything.

i would then argue that the provision of the box should be up to them due to the nature of its importance to receiving the service and that they should replace it. i reckon there'd be a good chance they'd agree, rather than make you fork out for a new one.

Do what Kimber says. Or get a a Humax.