You have an oil delivery issue, there's no quick fix it needs investigating. Unfortunately serious damage may already have been done if it is a "developed" habit!
"Clueless new Range Rover Defender owner-to-be..."
Don't worry the fact you call the Defender a Range Rover already confirmed to us you were clueless on them.
You need to pressure test the coolant system to see if it is losing pressure, if it is check for external leaks, if none then you probably have internal cracked head/injector sleave which can be confirmed with a sniff test.
I've never known any parts warranties to cover labour costs unfortunately. If the garage had supplied the part then you may have been covered by them but no garage will cover labour for own supplied parts.