So 1000kW/hrs for a whole house in a month doesn't strike me as to far
from the mark.

Thing is, what I may not have made clear is that this is literally for
one and a half storage heaters. One in the living room on at about
half charge rate for the full month and one in the bedroom on the same
charge for about 2 of the weeks in the month.

To me, that seems excessive.

Andy.