You have contracted them to service argon regulators?
What servicing do they (the regulators) need? "Sticker" (date) maintenance?
Surely you would be better off (cheaper) by simply replacing with new when (if ever, more like) they fail?
But I hear what you're saying about monopolies (and BOC are part of a larger - international - group, of course). No doubt alternatives are available if you look hard enough.
Personally, I would not tolerate the arrogance you describe. In fact, I would cancel the contract, and bring the work "in-house".