Concrete hearth for solid fuel fire

I am going to get a cast iron insert installed to replace our very visually unappealing fireplace. This will also have a wooden surround which I have already sourced. It may utilise a gas burner, or we might just leave it as solid fuel (it is primarily decorative). What I am a bit confused about is my preferred choice of hearth, and how to make it happen.

I don't like marble or granite. Slate is just about OK, but if it was for solid fuel it would no doubt need to be slabbed which I don't want. Also the cost of buying one of these options is going to be quite high compared to concrete.

I would quite like to make a cast in-situ concrete hearth, possibly coloured and/or polished. My fitter obviously won't be interested in doing this, so I will have to do this ahead of the insert being fitted, so:

1. Can I remove the existing fire, and any brick detailing making up the fireback etc and then just lay the concrete as far back into that space as possible, so that the insert would just be placed upon this at a later point?
2. The existing hearth sits in front of the wooden surround a little at each side, so it would be easier for me to pour concrete with the surround actually fitted to the wall. BUT - how would the cast iron insert be fitted when it needs to obviously be a little wider and also presumably higher than the size of the internal space of the surround?
3. What minimum thickness of concrete is recommended?

Have a look here that will give you some idea http://www.stovesonline.co.uk/stove-hearth-size.html

chappers said:

Have a look here that will give you some idea http://www.stovesonline.co.uk/stove-hearth-size.html

Click to expand...

unfortunately that seems to be purely aimed at stoves, rather than inset fires, and has no mention of concrete either