On Sun, 15 Sep 2019 05:23:57 -0700 (PDT),
wrote:

The easiest way of rewiring this for E7 or E10 is to replace the meter
so you might want to look at a smart meter tariff) which will have both
24-hour and off-peak-switched live outputs.

No the easiest way is to feed the CU for the heaters via the main
(switched) contacts of the time switch.

The OP also needs to look at his electricity bill to see if he is
still on an E7 tarrif. Hopefully he is(*) and essentially all that is
required is the above wiring change at the board and installation of
new storeage heaters. New high heat retention heaters will need a
permenant supply but with two rings and three heaters that should be
possible to derive from the rings. I suspect one ring is kitchen and
the other rest of flat. Put one heater on the "kitchen" ring, the
other two on the "rest of flat" one.

(*) All consumption during the off peak period, that the meter still
"knows about" from the time switch, will be at the lower price.
Without the large heating load this isn't likely top be economic,
broadly you need to use 2/3 of your total consumption during the off
peak period to break even on E7.

--
Cheers
Dave.