There's a hole in my floor....

Right, started my first bathroom reno today. Got the toilet, vanity and showerscreen out, followed by the tiles off the walls, and the wall sheeting off. Then I took up the old shower tray. Now, I was planning to build up the floor and have a low, tiled hob. But as you can see from my photo, there is a large hole in my slab.
Now, what do I do with it? (That's an orange bag stuffed in the waste pipe at the moment to stop me filling it up with my junk or tools).

Once I've hacked up the old mortar the shower tray was siting on, I guess I need to put down some sort of board to place my new mortar on?
What should it be made of? And how thick? I wasn't expecting this result, and so am totally unprepared for it!

Any help would be more than gratefully received........:confused:

You would have had an aperature at that position anyway when you took away the tray.
My first thoughts are that you chamfer the sides of the hole all round the aperature and then make a mould and pour a plug that will fit the opening, the plug would need a smaller hole for the new drainage setup?

But I am sure there are bathroom fitters around with other ideas, if not visit some forums world wide with your problem.

Thanks Jow.
I'm planning to either put concrete in the hole, or perhaps just filling it with sand, as it may well have been left like that for a reason. Then I'll probably put a big piece of Hardipanel on top, then mortar, waterproof, and tiles.
God only knows if this'll work, but hey.......