The problem Boulby as it they will not up Salt production, they produce what they need to as a by-product. I spent some time working on a salt crew up there, Salt mining takes place at the weekend, with the weekdays being spent installing support, conveyors and other services. At the time we were producing about 7000 tonnes a weekend.
If the salt roadways are driven to far in advance of Potash production the chances are the tunnel will close as the stress would not be relieved by the potash workings above.
In Cornwall we are generally in a better position than most for two reasons: -
1. We don't use as much as its milder
2. We have bulk deliveries, due to the distance from the salt producers, it is cheaper to bring it in by boat from Northern Ireland. Cornwall council took their 1st delivery in October, which was 4000 tonnes, that will generally last the winter, however should we have a prolonged cold snap, the council have time to react by placing an order in sufficeint time. This is how Cornwall ended up selling Salt to other parts of the country last year.