There are many reasons a solar system can underperform from something wrong with the installation or damaged panels or other equipment from the factory, to the system is working as should but, not being recorded correctly for one of several reasons, existing CT clamp fault or installed wrong way round, even doubling or more up on twited pairs solves many issues however, its not something l’d like to hazard a guess at from miles away.
The installer seems to think installing an Eastron external meter will sort it, which I assume they believe the system is under recording, so you have got to give them a chance to sort it initially. They may not know what the problem is hence the reason the part is out of stock, so they have time to search/enquire for a solution however, telling you over the phone rather than comming out, indicates they new there was a problem with your system imo.
It maybe worth getting advice of energy Scotland on your next steps and if you haven’t allready done so enquire if you can change installer for the battery, as its all been going on too long with too many iccups along the way.
I would be inclined to get my prostrate removed if I had it, sadly it runs in my family and my dad passedin December after a 6.5yr battle with it, but his was advanced when first detected because doctor kept forgetting to put the blood test on his yearly checkups.