Firstly you have in this case done your own basic risk assessment and found that the work is dangerous and you would like something done about it. If management will plead ignorence then you have to put it in writing and get your own copy - email, that way you have identified that it has been brought to the attention of those who have responsibility for safety. Not easy to do if you don't communicate by email. Second option is to come up with a solution and present to management, like a mobile phone for all lone workers, sign in and buddy phone call at specified times or whatever. the third option is walk - not nice idea I know but ask yourself - do you really want to be working for an employer who will not take on his duty to provide a safe place of work for you?