Geofix failed - best replacement

[Kalozo]

I have another post on here about patio slabs coming loose. I also want to ask a specific question about how to joint.

I am having a patio relaid (see other post). It was originally jointed with Geofix which has completely failed - crumbles and cracked. It has also gone black in 2 months.

When the patio is relaid I want to make sure the contractor uses a better jointing compound.

Patio summary:

1) 900 x 600 Indian harvest sandstone
2) About 65m2
3) North facing so not much sun and damp over winter
4) Has lots of trees near by with leaves falling on to it

What would you suggest using as the best jointing solution?

Thanks

[Tom - Westminster Stone]

For jointing use either mortar or gftk ncc 850

[lutonlagerlout]

I would say the best product I have used is GFTK 815
for ease of use and finish
LLL