I'm not 100% convinced the introduction of a beaver population will solve the problem. The geography of Cornwall is not the same as the Laxey Valley with it's steep sides and lack of a plateau anywhere along the route for them to build the lodges. And even 15 years worth of wood growth probably wouldn't be sufficient to supply an increase in beaver population given that their lifespan is only 10 years (offspring go off to find and build their own lodges).