Have a look at your copy of the memorandum of guidance to the electricity work regulations, which you should be fully acquainted with as it explains your statutory obligations under the EAWR 1989.
Here's the relevant bit to save you digging out your copy, with my underlining for emphasis.
Disused electrical equipment and systems
75 Before electrical equipment is decommissioned or abandoned for any reason it should be disconnected from all sources of supply and isolated. Isolation (as defined in regulation 12(2)) requires taking effective steps to ensure that it is dead and cannot become inadvertently re-energised or charged by induction or capacitance effects. (Regulations 12, 13 and 14 are also likely to be relevant.) Suitable labels or notices to bring people’s attention to the state of the equipment are likely to be necessary in preventing inadvertent re-energisation.
One way of complying is to connect all the unused lives to the main earth, which may be what you have, as others have pointed out.
