I would be putting a C2.
If it had been a TT then perhaps a C3 if it had a thick, deep, stable earth rod or perhaps mulitiple earth rods. But if your other Zs figures meet the required figures and this 'one' doesn't then I guess the earthing arrangement must be as you have stated (PME), so I would say a C2.
I find it strange when I put down a C2, or perhaps multiple C2's and the customer doesn't get back to you! This has happened twice and I know for certain that on one occassion they have not had the work carried out.