OP
Bobster
unfortunately you've been ripped off, there was never any chance of a 20% increase in generation unless your previous system was heavily shaded and using a poorly performing inverter in the first place.
Please complain to trading standards about the company who mis-sold you that 'upgrade'.
He wont get anywhere with complaining to trading standards, they stated 'up to' which is as good as saying, you're not going to get anything.
As for the performance issues, I've seen a 10% reduction in power production just with having a fine layer of dust on the panels. Fine this was on a site generating 80MW, but the principal is the same.