He took out the old card meter, installed a new key meter, and wired everything into the new twin rate meter. Turned out he was actually the guy who'd first rewired the system so that I was paying peak rate for my water and heating, so at least he vaguely remembered what he'd done back at the beginning of May. He assured me that the off peak stuff was working now, so I shouldn't worry until the evening now.
So, comes the evening and no hot water. Again.
I'm now back to the stage before I was paying double for my water and heating. The stage with none. Calling up the company, I'm greeted with "All our computer systems are down today. Please hold on if this is an emergency or our non-peak times are from Tuesday afternoons onward next week."
I may cry.
EDIT - Emergency engineer will arrive "in next three to four hours". Yay?
EDIT EDIT - Only one hour wait! *crosses fingers*