After about 30 mins of struggling to change the default install dir for inno setup I finally found the answer: Basically the previous install directory for software has precedence of the DefaultDirName, so you need to turn this off, and it is done as follows:
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UsePreviousAppDir=no
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Your software will now default to install to the directory you specify with: DefaultDirName={pf}{#MyAppName}Some sub folder
Happy Compiling and Cheerio ?
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