Repairing old hearth and filling large hole

[BenDIY2021]

Hi there,

This is my first post and have been searching the Internet for someone with a similar issue. I have opened up the fireplace and have removed an old metal Ash box which has left a large hole in the floor.
I am looking for the best solution to help infill the hole which is around 20 x 30 cm in width and about 30cm deep.
I have currently put bricks inside the hole to help pack but unsure on what I could fill it with to make the top level. There seems to be lots of rubble within the hole already.
The wooden floor is suspended and there is a large air space throughout the house.

Any advice would be appreciated

Thanks