the solar salesman told me that with 20kw panels, €150,000 investment, i was looking at a tax-free revenue contract income of €30,000 per year, and no more metering my own use.

Times like this, I regret living so far north: from what I can see, you'll collect about 50% more energy at 45N than 55N (or I'll collect 2/3).  And it would take a lot longer to recoup the investment.