When looking at the Regs:

"8.7.6 Elevated voltage on supply neutral"
With the main switch off, measure the voltage between the supply neutral
and any earth external to the installation. Notify the supplier if the reading
exceeds 25 V.
Disconnect the installation and notify the supplier (see annex K) if the
reading exceeds 50 V.

This test is to actually test if the "link" between N-E is still in tact. Think about it logically, Neutral is earthed at the Transformer, thus if that neutral and Earth link is anywhere broken along the line, you will have a voltage between neutral and earth when you test it according to 8.7.6..