Pointing between Indian sandstone slab and Aco Channel drain

[EdF_uk]

Morning,

I have laid a patio with an Aco channel drain at one end. The slabs next to the drain have about 5mm gap and are about 5mm higher than the side of the drain.

I have Sika Fastfix jointing compound to joint the patio. What is the best way to joint the drain gap? Obviously it needs to be filled properly so the water runs into it. Should I just work in the fastfix or use mortar? I have a mortar gun which I guess will make filling it easier.

Any recommendations would be good.

Thanks

Ed