I think what they want you to do is make a temporary earth point (new peg in the ground) "any earth external to the installation". This obviously because of the fact that many installations will have the N & E linked on-site, so testing the potential between them is a pointless exercise (unless you either disconnect them from one another).

But to me, testing if the PEN conductor is intact is much easier done by an on load voltage measurement. If the PEN conductor is high impedance, a volt drop will occur over it and thus a lower voltage across your load. If this then fails, additional testing can be performed to ascertain if the fault is on the supply or consumer side?