what I was hoping to do was to create a dialogue by putting them (the managers) front and centre into the situation so that it cannot be ignored - i.e. if you are happy for this to go on in your opinion, please ensure you take responsibility, are seen to be involved, and participate in it, with the ability to be called to account later on the situation.
in addition, the issue of relationship between the director /senior managers and hse manager could have issues until the director/senior manager can understand the the hse manager is going to do the best for the company, and trying to get things done in the proper way.
decisions and tests of resolve to do what needs doing in the right way always exist, however it is difficult to pick the battles you need to fight. in this case, I'd be inclined to "work together" to solve the problem, by shining sunlight onto the issue, and asking that they are happy to sign up to/take responsibilty for the alternative solution, rather than use of the mewp.
if cable tying new cables being run along a ladder, I can see working from above as the contractor being attractive, however at the same time, if I were on the ladder, I'd also want to know that it would hold my weight, and also couldn't really know how well the anchors were in in the first place, so the issue from a practical perspective could be "grey" to the contractor employees. (it's only 25 minutes what's the problem), just want to get finished to next job...
however there definitely needs to be a roles and responsibilites discussion on acceptable practices, unacceptable practices, and tolerances, if only to ensure that you know where you are pushing against an open door and what would be viewed as unacceptable.
as said before, it seems you are relatively new in the organisation and putting some markers down now would be likely to save time in the future and generate an open environment for all. perhaps also suggesting doing the iosh managing safely and directors health and safety to encourage all to consider it as part of their role.
we did it with all team supervisors, (so included others) to spread the safety love, and it created appreaciation of the issues, and more engagement/taking account of health and safety rather than "not my problem" - kicking to hse to solve, (and some not wanting to do the course), but people are people.