these are possibles
are you  DEFINITELY STILL ON  a duel tariff  supply
Can you CONFIRM   that the time clock  POINTER settings changes from (N) normal rate to(L) low rate  setting  on the time clock. The  storage heater meter supply does not become LIVE till on the(L) low rate  setting on the meter 
check the STORAGE HEATER(s) supply fuse box is turned ON. 
Check that the storage heater FUSES, in the fuse box. one for each  heater have not BLOWN 
if all of the above are  ok  its time to test with a DMM behind  the face plate with the neon on it for 230/240 volts.  If you don't feel able to do this, (not all people have the confidence to do it  )its time to call the experts in 
look up a sparky in the  local yellow pages
                
 wirecutterman
                
 November 2009