Concrete around flagstones, on sand?

[StretchArmstrong]

Hi,
Am very ignorant of flooring.

Placed flagstones on sand / soil.
But there's gaps.

Any suggestions on what I can fill the gaps with?

The flagstones are about 3-4 inches deep.
V.heavy, don't appear to me moving anywhere,

Thanks in advance.

 

[Ian H]

I’d use balloons but if they didn’t work a wet mix of grit sand and cement.

 

[StretchArmstrong]

Thanks Ian, as the balloons keep deflating, I think I will need to take your advice