Immersion boiler help? Which switch to use!

Hi, we live in an all electric flat and bills have been ridiculously high. There's two of us, both working full time and bills are in the region of £250.

We have an immersion heater which has two switches. One is linked to an economy 7 switch (the lower half of the tank) - which I presume was the old night time tariff and one is the top switch. We actually have no night time tariff and are just on a standard 26p per unit rate all day and night. An electrician told us that having it switched on all day (which it was) could be causing the high bills and has advised us to only switch on the top part of the tank as and when we need it and never use the bottom part. A plumber advised different.

Would anyone be able to explain?