THis morning, whilst checking the oil, I noticed the point where jump leads are attached was all melted around the terminal, to such an extent, that the post had retracted inside the plastic cowling and towards the battery.

Is this a common thing, or anyone else had this happen? 2010 MY Vogue SE TDV8 3.6 Stornoway Grey- fully loaded

There is a lot of heat generated when jumping a vehicle and if the clamps are not connected properly it will cause the symptoms you have

Especially when I connected up the cables the wrong way round. Melted cable covers though, not the connection point plastic  .
Experience is the only genuine knowledge, but as time passes, I have forgotten more than I can remember
Volvo V70 P2 2006 2.4 Petrol 170bhp Estate SE
MG Midget Mk1 1962

Previous: L322 Range Rover TDV8 3.6 2008; L322 Range Rover TD6 3.0 2002; P38A Range Rover V8 1999

A bad connection will generate heat make sure its all cleaned to ensure good contact and all terminals are tight. Shorting them out or connecting wrong like Joe will cause a large current that will melt the insulation off the cables and could set you Range on fire! Mine was a blue one!

If the plastic cowling is seriously distorted due to heat I would be inclined to suggest you consider replacing it.
The cowling effectively acts as insulation keeping the 12V starter cable bolt from the earthed body panel the cable bolt passes through. Any deterioration in the cowling risks the starter cable bolt shorting onto the body panel.
It's always possible that the bolt is already shorting intermitently causing heat that has affected the cowl.