I am off grid and installed a micro-hydro system to supply our house. It has required minimal attention and works 24/7, producing 12.5Kw hours of electricity a day, more than enough for our needs. These systems do work, it is only a shame that such plants could not be installed on the existing weirs for defunct mills, but the amount of surveys and licences required would make it too expensive.
The Milwr tunnel, however is a non starter as impounding it could cause major problems.