Thanks for replying guys, although, the two bits of info were a little bit conflicting:
Put it anywhere, then run bitcoin with the "-loadblock=/path/to/file" parameter.
bootstrap.dat goes into the data directory. The same directory as debug.log and peers.dat are located.
I think Jeff's is just the 'safe' way, or for if you are running an older version of Bitcoin-QT...
If anyone else gets stuck like me, what I did was:
OSX using Bitcoin-QT versions 0.7.1+
1) Open a terminal window
2) Open a finder window.
3) In the finder window, navigate to /Applications/ and the ctrl+click on 'Bitcoin-QT' and choose 'Show package contents'
4) Navigate to /Contents/MacOS/ and then drag the file in there ('Bitcoin-QT') into the terminal window
5) add a space into the terminal window, after the 'Bitcoin-QT' file, then type:
-loadblock=
So in the terminal window you will have something like this:
"Your-iMac:~ user$ /Applications/Bitcoin-Qt.app/Contents/MacOS/Bitcoin-Qt -loadblock="
Dont press anything else yet...
6) Next, with the finder window selected again, navigate to the location of your 'Bootstrap.dat' file
7) Drag this file into the terminal window, so that it adds it onto the end of the command, which will look something like:
"Your-iMac:~ user$ /Applications/Bitcoin-Qt.app/Contents/MacOS/Bitcoin-Qt -loadblock=/path/to/file"
Hit enter to run.
Hope that this helps someone else