Old fire place in 1st floor bedroom

[mersey_view]

Some years ago the chimney and then the chimney breast were removed to the 1st floor bedroom just leaving the old grate, which i have now removed. The hearth is a slab of concrete which i also wish to remove. The lower
part of the chimney breast downstairs is still in place although once the hearth has been removed that can also be removed.
The hearth is 'T' shaped with the lower part of the 'T' being supported by the
remaining chimney breast. The top part is surrounded on 3 sides by joists.
The house was built in about 1926. Does someone know how the hearth was
constructed in place and how it is exactly supported and how best to break it up to remove it - i don't want it falling thru the ceiling below as i break it up.