Loose patio slab issue - ideas to fix with slurry

[rh2205]

We have one loose honed sandstone slab & no spares, it was laid on a full bed but from tapping it all over one corner is a little hollow, & knocks ever so slightly when stepped in this same place, we have tried to lift it with hooks wedged between the joints & turned underneath, this worked on another slab so we could relay properly, but this one won’t budge so isn’t as loose. We have no spare slabs and don’t want to smash it out, honed sandstone isn’t a rustic finish so can’t lever it either with a spade without it ruining adjacent slabs, can anyone tell me the best mix to pour into the 5mm joint as a slurry to try & fill this hollow & get it to hold more solid before grouting, I wasn’t sure whether to use plastering sand as might be a bit finer as a 1:1 mix with cement, only have sharp sand currently. I will tape & protect all the nearby pavers to prevent staining. We did primer them & just need the best suggestion/tips for fixing this one remaining issue.

[Radian]

I used a roughly 50:50 mix of SBR and OPC to fix a similar issue. No need for sand.  Made it runny enough to pour in through a funnel after first flushing the joint with water. Have a bucket and sponge handy if you get some on the top of the slab. 

[rh2205]

We have some ready made still bagged paving primer left over, will that be good enough as an sbr equivalent & I will add in some more opc as a 1:1?

[Radian]

I have no idea what's in paving primer. SBR is a latex solution - pure magic in mortar mixes and slurries.