After a further 22% slide in Energis shares today, the group's value has fallen to £313m, leaving National Grid with a potential hefty write-down against the value of the Energis stake. A FTSE 100 stock, National Grid is valued at £6.7bn. National Grid shares moved down 10 3/4p to 442p, with Goldman Sachs reducing its fair value for the shares to 455p from 470p. Analysts estimate a 10p movement in Energis shares implies a 3.8p movement in National Grid's stock. Two National Grid executives, Stephen Box and Edward Astle, are nonexecutives on the Energis board.