Dear Sir/Madam,
I have reviewed the jpg's attached and from what I can see - the concrete slab is essentially unviable. The exposed rebar at the underside indicates to me that the steel is completely compromised (it has no concrete cover) and likely it would be more beneficial to cast a new slab in its place.
If more information could be provided (plans and sections of the cellar and slab, with dimensions - hand drawn sketches are fine. Also any information about the masonry walls and soil in the area would be beneficial), I could help with this task.
Regards,
Conor.