white meter heating times?

On my 'boost control unit' the yellow light used to come on at night and if I wanted hot water I'd switch the 'water heater' switch to 'timed' - this would then heat the water overnight.

However now the yellow light seems to come on at a random time - mainly during the day - if you switch the 'water heater' switch to 'timed' the water does heat up but I'm assuming it's more expensive as this is during the day.

How can I get the water to heat overnight again?

Hope someone can help.

Thanks