I use the 13A mains charger, and normally charge overnight on economy 7. I have set the charge timer period to begin at 2 AM, and finish at 8 PM. A full charge takes around 5 hours, so it's finished by 7 AM, within the cheap period. But I can pre-heat at 8 AM (using the App), and draw some of the power from the mains. And I can charge at any time up to 8 PM - this is useful for me, as, if it's sunny, I use power from the Solar PV array on the house roof, instead of overnight.
I simply have to ensure that I don't activate the charger until AFTER 8 PM if I plan to charge overnight, which will always be the case for the next few months.