Thank you again Tim! I was considering connecting the negative to ground, but was very wary of doing so...
Finally I have been able to get through to 'Master Power' the branded name of the Axpert inverter I have - unfortunately I can't parallel my inverter
So the way I see it is that I have 2 options - 1. is to use a Victron Orion Tr24-48 to give a 48v feed to a pair of Growatt inverters and the other is to try and connect my two 24v battery banks in series and buy a larger 48v hybrid inverter - The battery banks are of similar capacity (one being built from 18650 cells and the other LEV50s, but no way identical) the BMSs are generic ANT ones from China and so far I have been unable to find out if I can connect them in series . . .
So that's where I'm at