I read somewhere that the West Country pronunciation of oi for words like fight and like would become foight and loik. Was this really more common in the 17th and 18th Century?
The Great Vowel Shift (roughly 1450-1750) accounts for these types of changes. The remnants of the pre-shift changes are often the results of some form of isolation (i.e. a lack of exposure to the shift in the general language population).
Though the GFS was primarily in England, the results of the old vowel remants are even observable in the US in cases where immigrants from isolated regions of England remained isolated from the general population.
There's a lot of good information on the web (stick with universties when possible) and some great maps and diagrams as well. These will help to visualize where the changes were most aggressive and areas where some of the older unchanged sounds are still common.