Old warm air system removal

mitzy

I live in a 1970s built terraced house that had warm air heating installed from new. The boiler has long gone and been replaced by a gas boiler in the airing cupboard. The compartment that housed the boiler is still there and I would like to access the chimney to use for a wood burning stove. It is a large 1 Mtr square structure in the centre of the house and if I stand in it there is a steel plate above me. There are some large holes in the plate an above that I can see large concrete slabs and the chimney opening in the corner. Could someone tell me what is likely to be behind the concrete slabs? Are they supporting anything etc. Any info on how this may have been constructed would be most helpful.

Thanks