Hmmm. That sounds like it might be beyond my skills no matter which
tool I use. I'm afraid of screwing up the slab the house is built on
if I attack it with something like that!
Any ideas for raising the rest of the floor to the same level? I
thought perhaps installing a plywood subfloor around the uneven bit,
but since the old floor is just those 1960s plastic tiles glued
directly to the concrete slab, there isn't really anything to nail it
to. My father suggested I glue sheets of plywood directly to the old
tiles using tile adhesive but I don't know if that would work or not.
Can you see if the concrete floor is actually under the walls? If it isn't,
then you might be able to remove the whole slab and level the floor off
throughout. If it is an old coal bunker as you say, they were normally
poured concrete over timber joists. To see if your floor is like this, you
will need to lift a section of floor boards to make sure the joists run
directly under the concrete. If they do, then the concrete slab should be
easy to remove and the joists and floor fixed to match the rest.
To remove the concrete should be as simple as drilling a cross of holes
through the concrete, and break it into pieces with a big hammer. It sounds
like hard work, but you are guaranteed a lovely finish to any floor
coverings you lay in the future.