concrete floor query

ive been called to look at a suspended timber ground floor which was springy. cut a long story, taken out rotten joists, thought of replacing but brick on which bearer for joist was on is unstable so id have to set those again, so think it might be simpler to concrete slab.
1/ do i need hardcore or can i just beef up concrete, otherwise i've got to take more soil out, hardcore in and tamp down.floor is not loose and has been sitting there for 100 years.
2/ whats the depth of inslation req? is it still 50mm?
3/ ive laid a few b4 without a screed. is ther any reason it could be a neccesity apart from harder to finish off the concrete,

MOD

moved as posted in wrong forum

To be honest, it's quicker & easier to do suspended floor than concrete floor.
Another problem you may have if using concrete floor method is the air-brick vent will be blocked therefore preventing air flow to other rooms.

i've gone back to that method while thinking it over last night.
il just have to trench out where brick supports are, and concrete with a section of dpm under.
much more fiddly, but i agree with you.
thanks for your thoughts.