Pauls Extreme Sound Stretch is just what I've been after. I just needed to do a bit of maths to work out the correct factor to stretch my audio file to the desired length. If you want to do this accurately, you have to do this:
Subtract twice the length of the window from the length of the original file. In other words, if you stretch a 1 second file of 44100 samples by a factor of 100 with a window size of 1024 samples, the resulting file is actually (44100 - (2 x 1024)) x 100 = 4205200 samples long (~95 seconds), rather than 4410000 samples (100 seconds).
A bit nerdy, I know, but useful I hope!