If I'm measuring a Zs, I'll usually do the Z(line) at the same time; it's an easy check that there's a decent neutral connection. It's just another button to push on the MFT if you've got three test leads connected already.
(BTW, what's the proper symbol for line impedance?)
Nowt in BS 7671 on the subject of the Line-Neutral impedance ... another sign in my book, that the regulations focus so much on the safety function of an installation so as to leave a gaping hole in the safety of the functional purpose of an installation, to distribute power!