you could always try quoting reg 28 of the electricity safety,quality and coniniuty regulations 2002.
this regulation requires distributers to provide the following information to relevant persons free of charge:
-the maximum prospective short circuit current at the supply terminals,
-the maximum earth fault loop impedance of the earth path outside the installation (Ze),
-the type and rating of the distributor's protective device or devices nearest the supply terminals,
-the type of earthing systemapplicable to the connection,
-the number of phases of the supply,
-the frequency of the supply and the extent of the permitted variations,
-the voltage of the supply and the extant of the permited variations.