I have just started to get a rubbing (?) noise from the rear of the car when braking.

What is the most likely cause?

The car is booked in for its 4 year service in 10 days so I can get it fixed then, the car has done 43k miles so far but it would be good to know if the wallet is going to take a big hit.

Could be the pads have no material left on them and its metal on metal, are you a heavy braker?

Sounds like that but I'm not a heavy braker

However I bought the car one year ago so maybe it is just time for them to be replaced anyway.

Mine is a MY13 like yours, just had a service at 36,000 miles with an advisory that the rear pads and discs need repacing within a few months, and I'm not a heavy braker. Booked in for new ones beginning of April.
Sheet said fronts 30% worn and rears 80% worn, strange why the Freelanders brake set up is for rears to wear the most.

Sounds like disc pads worn.

But also consider (especially if your handbrake isn't working too well ):
Brake Shoes which provide the hand brake unction and work in a drum which is integral with each brake disc.

1)the adjuster linkage seizes up inside the drum.
2)Also he drum gets rusty and the roughness causes shoe lining to break up
This seems to be more of an issue with automatics as the hand brake doesn't really get much use ( advice here is to use the handbrake on an automatic to pull up the car (gently from 15 mph or so ... common sense precautions apply) every few weeks or to skim off rust and keep everything working properly.

Probably more likely after the ravages of winter road salt as well

Best to dismantle and have a look.