Lurcher, we have a similar problem or at least did. A marine charger with a decent output will be £300. Have a look at Sterling Products if you want to to go that route.
Our yacht sits on a pontoon but without an electrical supply, so for while we had a Honda genny to give us sparks, heat via oil filled radiators and power for 240v tools. A transformer gives me 240v from the batteries for most things including the slow cooker
Then I started buying solar panels. We now have 3, two fixed and one portable which can be moved around to face the sun. Total 180w which gives us enough charge during the summer to sit at anchor for a week without running the engine. Since the genny wasn't being used that was sold off and haven't looked back since.
As a final benefit I have noticed that anodes now last two years rather than one because we don't use shore power. With that big flat roof I would be looking at a nice big solar panel and a PWM regulator which will work all year round. Good time to buy too