Vacuum Packed

Slab laying well under way, and that wee bit of plant in the foreground has been a revelation!
It's a vacuum slab lifter, making the 35kg 600 x 600 slabs a two person lifting pleasure.
So, we screed a 600 width of sand and gently lay each slab in place, a few taps of the mallet, and on to the next one, seems to be working well, thou stability will be put to the test over time.
Now thinking what to use for jointing?
Requiring security with some flexibility.