Ok thanks so if I go out in the morning tap that back off, get something like that adhesive.I do not think the small pieces on top will work, it may but it is iffy. Your 3 to 1 concrete mix is better for doing thickness work like a footing or formed walls not laying stone like that, you would need to add lime to the concrete which would bond a little better. Any good adhesive would work that bonds to masonry. Here is the US the PL product line is good. Here is a picture of what I would use if you can get it.
View attachment 111067
If I put that adhesive on the concrete I've made and then put the slab on, will it instantly grab the slab or will I need to wieght it down again?
The slab is very heavy and easily tips off the edge of them blocks I've made.
The top of the blocks are only 15cm wide I struggled to get it to stay in place with the concrete without leaving a large section over hanging inside the pond.
I'm across the pond from you in the UK.
Would this be similar to the adhesive you sent a picture of ?
https://www.screwfix.com/p/nemesis-...gclsrc=aw.ds&dclid=CMGFpMXGmtsCFcwS0wodWOwFzg