I can understand pulling the fuse to connect new tails, but opening up the live cutout to connect an earth into a service as old as that is a bit much in my opinion.
I would agree,and even though the person supporting self-involvement,is unarguably,a knowledgeable and experienced fellow,the ongoing risk of responsibility,is a problem.
I suppose,you could say,if the DNO were asked and gave you in writing,the permission of doing such a task,then the worry has gone.
If you know this is not the case or will not happen - then there is your base-line legal premise,for it possibly being a risk.
I will add one more thing. The investigations,which throw up issues of this nature,are often instigated,due to another persons error,or a random or unconnected incident.
The size and seriousness,of the incident,equates directly,to the depth and quality,of the investigation. Any,and i mean any,deviation from the rules and regulations,present at the time of the work,will be used as a reason to obviate liability,and therefore compensation.
A classic example of this,was the gas explosion,in Eccles near me,some time back,where the plumbers/heating engineers,had some of their work held up as non-compliant and possibly dangerous,even though it was demonstrated as standard practice,known to the gas authorities,and still carried out today.
An incident of such seriousness,which would bring one's own work in to scrutiny,would be a one-in-a-million deal...but we imagine what we would spend our lottery money on,and that's much greater odds.