Creating a gap under hearth for air flow

[PJMe]

Hi, this is my first post after taking on a "project" house. It probabaly won't be my last!

I've opened up a chinmey to the builders opening which will (in the long-term) house a gas-fire (stove style most likely).

As per the photos I've removed the hearth which was, I think a 1970's replacement as think and broke up. Iv'e also dug out the soil which was above the slate DPC. The DPC goes all the way around the wall and hearth wall. All timbers are in good condition.

I'd like to retain a gap below the DPC for damp reasons and build the hearth back up. How should I do this to meet the regs and allow a gas fire to be installed?

I'll also want to put the gas pipe in capped off in preparation.

I was thinking of either:
1. extending the joists into the hearth and laying slate/slabs onto which I could lay concrete
2. filling the gap with rubble to allow air flow and somehow pouring concrete over it without it trickling through the gaps (don't know how that would be possible!

Any advice would be greatly recieved.
Thanks