Thanks for the reply, there is no reason I cannot power off the machine whilst this is happening.
To be honest, this crossed my mind when I first started reading up about the procedure but every guide I have read seems to do it as a 'powered on' machine so I kind of presumed I had to use this method.
I'm just running though a test at the moment with the machine powered off, I will let you know how I get on (it seems to be a bit quicker as well)