Christian McArdle wrote

It is just the 1.8m section that will only be supported
on 3 sides that I'm worried about.

etc

I think this is far too long Christian. I bought a piece of 20mm granite 2m x
300mm for the shelf over the concealed cistern in my Bathroom. I collected it,
put it in the back of my estate supported on 3 cross battens, wrapped it in a
blanket and tied it with a rope. I drove home like I was following a funeral,
but when I got there I found the bl**dy thing in two pieces! I was really
surprised how weak granite is in tension - the strength varies with the type of
granite and the size of the quartz particles apparently.

I would say you need to talk to the suppliers (and get them to deliver it!)

Peter