BF I calculate as follows:Average trench drainage rate (from...

 BF I calculate as follows:

Average trench drainage rate (from latest update)- 1.62 cubic metres of brine per linear metre of trench per day = 1.62 x 365.25 = 591.7 cubic meters per linear metre of trench per year. 

PFS requires 65-68M cubic metres of brine per year.   

68,000,000 / 591.7  = 114,923 linear meters of trench = 115 linear km required to supply indicated requirements.  Call it 150 -200 linear km of drainage trenches required.    

Could you explain the justification for saying "They will need wellnin excess of 500km of trench on those figures"  ? 

PFS has requirements for 546 linear km of trenches including feed trenches to supply the 65-68 million cubic metres of brine per year.  So it looks to me like the PFS which budgets USD28M for construction of the trench network is  appropriately conservative.   In any event at 7% of projected PFS capex the project shouldn't be that sensitive to initial trenching requirements. 

At 3,633 mg K / L or 3.63kg/cubic metre of K (molar weight ratio of 78:96 K2:SO4) gives a sulphate of potash concentration in brine of 8.1mg/l  which is bang on the PFS figures of 7.7 - 8.1 mg/L.

Previous field work had similar concentrations and average flow rate of 1.4 cubic metre per linear metre per day of trenching. 

Looks fine to me.