Constructional hearth for fireplace - Best way to construct?

[shimmy]

Hi all. Newbie here, but I have read much helpful advice on these pages for which I am grateful. Here's my query. I have taken up a suspended rotten timber floor and before installing a new one I need to build a new constructional hearth in front of the fireplace (the old one was not up to regs, cracked and built on loose earth & rubble which more or less collapsed as I got to it). I am aware of the regs requirements for a real fire for the dimensions of the hearth, but as the overall depth from ground (earth) to the new ffl is 418mm, I need your advice as to the best way to make it. My thoughts are as follows:

New Constructional hearth, 1400mm wide by 900mm into recess. (regs for hearth thickness 125mm non combustible)
Overall height required from ground up (earth).. to level with new ply floor is 418mm.
I intend to dig a shallow 100mm x 100mm trench in the earth and fill with concrete for a little foundation, then build a lite block retaining wall and fill that with the appropriate materials. Can you advise me on the different depths required please for
1/ Hardcore
2/ Sand Blinding to take DPM over.
3/ Concrete (MULTI USE READY MIXED I have at mo)
4/ Screed (if recommended) 3:1 sharp/cement?

Obviously I would want as much Hardcore as possible to keep the mixing down on the others...what would my min thickness's be please.

Do I need to finish the hearth with a screed or will straight concrete be ok. The decorative hearth may not be fitted for a while so the constructional hearth will be seen for a while yet. Will the concrete finish look a little crude, or is this down to the aggregate size. If screeded, should it be bonded to the concrete and with what?.
Or...am I over complicating the whole thing with my little knowledge!
Thank you all for replies guys.

[lutonlagerlout]

dig the little footings according to the sub grade
but i always aim for at least 200 thick by 200 wide
then block up
I am not a fan of hardcore unless its well crushed and compacted

alternatively just shutter and pour concrete in the lot for a bullet proof job

LLL

[shimmy]

Thanks Lagerlout. I take your point about the 200mm foundation. If I use your second suggestion, ie...fill all with concrete...do I need some reinforcement in the concrete as overall I have a depth of 418mm, though I suppose I could do 350mm of concrete with a 68mm Screed to top it off. What you think?

[lutonlagerlout]

350 of crete is not excessive
obviously still use your dpm
cheers LLL