For anyone without a Zappi charger, I've been using the Jedlix app and that seems to work well. You link it to your car, set your cheap rate period and it just gets on with charging the car starting and stopping so you only charge in the right window - although you can override it if you want anyway.
I have it set up to immediately charge to 30% if the car is lower than that when I plug in as well. In fact the only real problem I've found is that despite allowing you to set the desired charge level in the app it always gets overridden by the car's settings.
Sadly they're not currently partnered with any energy companies in the UK, on the continent they will charge your vehicle any time prices are low - if there is an excess of solar mid afternoon for example. Then again, that's not a scenario that's likely to come up here any time soon anyway.