Repair or replace concrete hearth?

[juniperdiy]

I am reinstating a decorative cast iron fireplace and tiled hearth in our bedroom in a Victorian terrace. I've removed the plaster from the chimney breast and exposed the builders opening. I'm now wondering what to do with the hearth...

- It seems like a layer of concrete over rubble, then what looks like a wooden base underneath
- It's level with the floorboards at the moment, which doesn't work for us as we want to tile it so it's flush with the floorboards (we won't be using it as a fire), so needs reducing in height
- There used to be a radiator in front of the chimney breast so a pipe has been laid into a channel in the hearth (which will be removed). That's the biggest area of damage but the other corner has a fair bit of cracking too.

My questions are:

1. Should I rip it all out and pour a new slab to the right height? If so, I understand that I'll need to pour concrete then self levelling compound on top - is that right?
2. Alternatively, should I chisel it down beneath the level of the floorboards, then fill the gaps, and use self-levelling compound on top of the whole thing? If so, what should I use to fill the gaps?
3. Do I need to be worried about destroying the downstairs ceiling through either chiseling out what's there or pouring new concrete?
4. If pouring a new slab, do I need to make sure it's supported in some way so we can walk over it?

Any help or guidance on this much appreciated!

 

[cdbe]

Usually sitting on a platform of floorboards halfway up the joists. Probably the easiest and cleanest is to pull all the concrete and rubble then build back up with timber and finish with 18mm chipboard, plywood or floorboards (if you're not doing other work then whatever you can get hold of in a small quantity) followed by tile backer board (about £7 from Wickes).
This is the last one I did (the hearth was just the bit on the left - the rest is where the chimney was removed).