You're spot on John, what Potash is currently seeing, is a rerun of the International Tin Council Collapse, which killed off the Cornish Tin Mines. The price of Potash has dropped from its peak of a £1000 a tonne two years ago to £350-£400 a tonne, following a Russian Major producer pulling out of the cartel last week.
It will take a while to even itself out again, although it will not take as long as tin did (Nearly 20 years) as the stockpiles aren't there.
For Boulby, they're still quids in - in 1999, the price was £120 a tonne and they were doing very nicely then.