Welcome.
You will need to check the specifications of your panels and inverter. From an 'overpanelling' point of view, it is likely a 5kW inverter will support 7kWp of panels.
However, you will need to check the max voltage that the inverter supports and see how that relates to adding more panels to your input strings. You will need to calculate max voltage possible at lowest winter temperature in full sunlight - so maybe plan for -10C worst case scenario for Cornwall? You will destroy your inverter if the max voltage is exceeded.