Concrete floor raised at one end

I have concrete floors in my 1960's semi. I have pulled the carpet and stripped off what looks to be the original lino tiles. Towards the patio doors the last 500mm of concrete raises up by nearly 30mm in a slope and sounds hollow. There is a small crack where it starts to lift. I broke a hole in the floor next to th door and found a large gap between the floor and the rough concrete base underneath.

Any idea what could cause this? I am thinking of excavating a cross section but wonder if anyone has seen something like this before?

Thanks

Graham

I have seen it - in Sussex in 3 places - Manor Park Uckfield , East Hoathly and a Bell+ Pearson house in Hailsham the oversite concrete is sinking because it is on hardcore that`s not compacted correctly when built . You`ll probably be able to claim on your buildings insurance

Well, there's a surprise lol. What do you make of the picture below? The big gap I found turned out to be the cavity wall gap... On the other side of the doors you see to the left is a recently built conservatory. It actually doesn;t appears to have dropped by much at all although it is a little hollow sounding in some areas round the edge of the wall.

BTW Nige F, if I need it what line of trade are you in? Presumably the sort that can fix these sort of problems I have a kitchen going in in five weeks lol.