Hi @Whitwop , it’s usually to do with the switching speed of the inverter. Once there is a feed into the house from the solar, if there’s a rapid change in irradiance say clouds for example, it can take a while for the system to switch in the batteries to consume the excess. 
 

I’m saying that, presuming that the inverter priorities are set correctly in terms of feeds and levels

Thank you. Any idea how I can find out the correct feeds and levels? There appears to be a dearth of proper instructions out there! 

There is indeed … no joined up expertise between electricians, installers and support companies. 
You could start by looking at the manual for the inverter - guess it’s a SolaX of some sort. 
It will look something like this but you should explore the correct version for your set up

https://www.solaxpower.com/uploads/file/x1-hybrid-g4-user-manual-en.pdf

Thank you very much.  I have a direction to go now! 

No problem. Pop back here if you need anything else .. there’s a few now with solar and battery systems who can help. A few with some SolaX experience too (mines a LuxPower but operates in a similar way)