I would contact the DNO and tell them that there seems to be a problem with the earth that they have supplied. If they ask for details just tell them that it appears that the connection may be loose.
I've tried by enquiry to find out what the earthing system is used at properties in the past to no success as they apparently do not hold these records??? Now I find that bizarre considering they have a responsibility to maintain their equipment. Now if they provide an earth then they'll have to attend... They'll soon tell you if they do not provide an earth to that property.
I would play dumb in this instance and explain that they have definitely put an earth on the sheath and that it's a TNS supply. I would not tell them anything about it being an earth clamp, just mention the clamp looks loose and see if you can at least get them to attend.
If they know for a fact that they do not supply an earth to that property then you'll have to explain to your client that they will need to pay you to install a rod to make it a TT system. Failing that, walk away. Providing you have not already commenced work in the property because if you have you'll be in the s*** as it will be your responsibility!