Thegie

You also get solar powered circulation pumps that are pretty cheap. They run of a 10-20W panel and cost a few hundred rand. Like this : https://www.sustainable.co.za/its-ts510pv-12v-dc-circulation-pump.html#:~:text=The ITS 12V solar DC,single family solar thermal systems.
Just run it independently of you controller etc in parallel. So if the sun shines it runs, otherwise not. I take it you have no thermosiphon due to the collecter being at the same level or higher than the tank. 

Depending how your backup system is installed (e.g. a separate DB with the essential loads) you could probably just have the control unit and pump moved to the essential load side so no switch in needed. Alternatively a transfer switch of some kind should be an option. Most likely in both cases you will need to get an electrician to run some wire if the pump and controller is wired up as explained by @Louisvdw