Yeah I figured the cost part would be the case.Nope, sadly this is a huge issue with installs where the installer doesn't do their due diligence and ask the right questions of the customer or inform the customer about the limitations of the system design. Sometimes I think they still think they are only installing a solar setup based on FIT, not on modern generation, consumption and battery storage combined with smart tariffs.
You could contact your installer and ask for a price to swap out the inverter and have the DNO G99 updated, probably wouldn't be worth what you'll spend save for at least a decade.
It's fine - I mean it would be cheaper to just replace the shower and plumb into the oil combi boiler so no high power outlays!
I was wondering if a second inverter and battery might help though?