Here is the basic problem: the air in your home is heated and warm air can hold more moisture than cold air. The moisture held by this warm air can come from a number of sources: leaks and wet foundations, cooking, washing, breathing... Some of these moisture sources can be controlled. For example, a good gutter, storm drain perimeter grading system may help your reduce the amount of moisture that enters the home through the foundation. Bathroom fans can help you expel moisture from showers and bathing. But all living activities produce some moisture and in almost all cases the moisture content of the heated air inside a home will be higher than the cold air on the outside.

See: http://www.soundhome.com/article/moisture-problems

In an ideal structure, a complete vapor barrier on  the inside of the wall and ceiling insulation prevents the moist air from reaching roof tiles and other exterior surfaces. If this moist air does reach these cold surfaces then condensation will occur. In your case, it sounds as if the warm and (relatively) moist air reaches the roof tiles. The temperature of the tiles is bellow the dew point of this moisture laden air and this is causing the condensation.

Old and drafty homes didn't have many problems like this. The moist and heated air simply escapes through the many holes in the walls and roof. The biggest problem is with partially and incompletely insulated and weatherized homes. It is in such homes that some moist and warm air makes its way into the wall or roof assembly and causes the types of problems you are experiencing.

Roof vents are a partial answer but even with a well vented attic space, some of the moisture will condense at these surfaces. The insulation may actually complicate this matter unless is has a complete vapor barrier on the warm (inside) side of the insulation. BTW, if this is fiberglass or similar material and it has been wet then it has lost most of its value and needs to be removed.

Dehumidifiers are also only a partial answer. They work well in enclosed spaces, spaces with an enclosed volume of air. Otherwise you are attempting to dry out all of the air around Devon and beyond.

The only comprehensive way to deal with these moisture problems must be based upon a systematic modification of the home's exterior envelope. This envelope must be waterproof, insulated and contain an interior vapor barrier. You will also need bathroom and kitchen fans plus roof and crawl space vents. This maybe a big job and you may need some expert local analysis. In many areas such help is available though your local gas and electric utilities or local government.