Found a quick fix in the MAMP forums.
Basically it seems MAMP is only allowing 2 versions of PHP to show up. Quick fix, rename the folders you're not bothered about using, for me this meant adding an "X" to my /Applications/MAMP/bin/php/php5.4.10_X folder. Now 5.2.17 and 5.3.20 show up in the mamp prefs.
Edit - if the PHP version you require isn't in the PHP folder, you can download the version you require from http://www.mamp.info/en/downloads/
Edit - MAMP don't seem to provide links to the alternative PHP versions on the download page any more. Use WayBackMachine https://web.archive.org/web/20180131074715/http://www.mamp.info/en/downloads/