Whats the best way to Fill a sunken garden?

Hi, total newbie to lawn care, landscapes etc....

My friend just bought a house, nice garden, dodgy patio.
Now she wants to fill in the sunken garden, about 2 foot deep, roughly 80 square feet, in a odd bendy shape. With a rockery and 2 foot high wall already inside the area, to hold a dirt border, with the existing grass 2 foot under patio level.

Whats the best way to fill this? She has the vague idea of getting a load of dirt in bags from the garden center and just filling it, then cover with a layer of turf. Im thinking better to get rubble, fill it in, then add a layer of dirt and turf.

Would the dirt only option cause subsidence, and is it even workable?
oh, and should i take down the 2 foot high border wall?
Should have added, im in the UK,
 
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