Filling large hole in concrete floating floor
This is how I ended up solving it, maybe this will help someone in a similar situation out.

After talking to the building regulations guys, I ended up fixing this hole using alternative 1 above (more or less).

A 40mm concrete slab was cut and placed in the hole, standing/jumping on it seemed very solid.



Added 80mm insulation on top of the slab and in the gaps on the sides so the concrete doesn't just run down into the hole.

Ten put down a damp proof membrane sheet before pouring 40-50mm concrete on top (no mesh needed). Building regulation guy suggested 60-70mm thick concrete on top as that would not crack as easy, but 40-50mm was fine