Two questions really: Our semi (c.1962) has a concrete slab outside the
front door, approx 2m by 1.5m. I feel I ought to replace it ...

(1) Not only is it very grotty by now, but it has sunk by about 3.5cm,
away from under the front step. (If you see what I mean: the front step
is of course a big concrete block which presumably is built into the
walls; you step down on to the concrete slab; the slab was originally
laid flush up to, and under the front edge of, the step; there's now a
3.5cm gap between the top of te lsab and the bottom of the step.)
Question: should I be worried that the slab has sunk by such a depth
over the years?

(2) Question: If I just break up the slab in situ, and possibly remove
an inch or two of the rubble, can I save on new concrete by just using
it as (rather deep) hardcore?

Cheers - (and thanks for all the inestimable advice gleaned by us
lurkers!)
John