After a quick read through these forums I see I am not alone in receiving EXACTLY the same obnoxious and distressing treatment regarding my late father's account when I contacted these people by phone earlier this week.
Not one iota of help was forthcoming from the indifferent oath on the other end of the line unless I could provide a "Plusnet username" and password. How the hell am I supposed to know these? And also, aren't passwords supposed to be known only to the original user? So I fail to see how providing that would be of any use. Unless your security algorithms are not all what you make them out to be.