Mixing mortar for paving (stiffness problem. oooer missus!)

I laid some paving slabs yesterday.

Used the "paving expert" recommended formula of 4:1 soft sand/cement, but really struggled to mix it up to be sufficiently stiff. It simply would not "flow" properly in my trusty 20 year old Polish mixer....unless I added too much water.

I guess perhaps there is excessive clay content in the sand

Am I missing a trick? Will plasticizer help? I also recall Tommy Walsh on TV's Groundfarce using sharp/soft sand mixes for paving mortar. Is this a solution?..

Any wisdom on this subject appreciated before I launch iinto phase 2 later.

.....

In consolation I spent all afternoon chuckling about the Monty Python Cheese Shop Sketch. "It's a little bit runnier than you like it sir!"