Raising a patio

[smiller]

I am currently in the process of laying a patio to the back of our house. I need to raise the level by around 20-25cm & the total area is 5mx2.5m.

I have built a concrete block wall around the edge to enable me to fill the gap prior to laying the slabs and my question is what should I use to fill the gap.

My concerns are that the weight of the filling will break the wall and so I guess I need to concrete this ?? - I was hopibng to just buy loads of sub base/hardcore and fill it up but am not sure if the wall will hold the weight of this.

Can anyone confirm the best way forward (What to use and how much is needed ??)