Over Sea: Just where is Trewissick?

Over Sea, Under Stone is set in a fictional town, Trewissick. When Susan Cooper was a child she used to take summer vactions in Mevagissey - a Cornish coastal town. Mevagissey is close to the real town of St. Austell, which is mentioned in the book as being close to Trewissick.

Below is a map of England, which to me kind of looks like a bagpipe without all the pipes sticking out the top. Cornwall is that pointy bit down in the bottom left. If England was a bagpipe, Cornwall would be the bit you blow in. 

 

So here is a map of Cornwall. Mevagissy (Trewissick) is mid way along the south shore:

 

 

So now you know!