If any OC Eng Wg is convinced that engineering airworthiness is not up to it, then he already has the authority to ground the fleet at his location. There is a QR (is it QR 640?, I can no longer remember) concerning airworthiness. This QR is the formal authority which OC Eng holds for airworthiness responsibility, and it enables him to delegate powers to sqn SEngOs. If OC Eng decides, he can withdraw his authority for his engineers to operate. Hence the fleet is grounded. He can do this without the say so of either the Sqn bosses or the Stn Cdr (although he would of course advise them first!)
This happened once I recall at a fast jet station. The engineers on one of the flying sqns were 'in difficulty', so he grounded that sqn. The other flying sqns were unaffected, and in fact they gained the aircrew from the grounded sqn. About 10 days later, OC Eng Wg allowed them to start work again.
At Kinloss, his authority is delegated to Nimrod Line Sqn (is it still called that?). So if he withdraws his authority, then all the flying sqns are grounded, not just one of them.