In article ,
"Roger Mills" wrote:

John wrote

(1) Not only is it very grotty by now, but it has sunk by about 3.5cm,...

Is it still moving? [You can tell by accurately measuring the gap a number
of times over (say) a 6-month period.] Has it sunk uniformly?

Yes, I guess it is. I suppose it has sunk about 2.5cm in the 16 years
we've been in the house.

If it is not moving, and is level but just too low, I would be tempted
simply to lay some decorative slabs on top of the concrete to make up the
level. Might even look nicer! It will be a lot easier and cheaper than
starting from scratch - and you won't have lost all that much if it fails.

Good idea, but I'm [now!] aware of the advice proffered by BigWallop,
which was that adding more on top will just make it heavier and cause it
to sink faster!

I might grit my teeth and get some quotes for having this replaced, and
also re-doing the drive, with which this slab connects (via another
slab).

If I were younger and less weedy (I'm a "slim 55 year old") I'd have
had the whole lot up by now and be hand-mixing piles of concrete! But
one of the lessons you learn on your way to 55 is that ... concrete is
very heavy!

Cheers
John