Topic: autotest - garbled output with numbers

I recently reinstalled my gems for autotest, and since then I have
unintelligible output coming from autotest like this (note everything
after unit_diff -u)

loading autotest/rails
/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby
-I.:lib:test -rubygems -e "%w[test/unit test/unit/test.rb].each { |f|
require f }" | unit_diff -u
76111971001011003211511710511610132451011083116971141161011001046107010511010511510410110032105110324846485255565332115101991111101001154610104932116101115116115443249329711511510111411610511111011544324832102971051081171141011154432483210111411411111411510

Strangely I have run my tests individually and they work fine.
I have then tried running autotest in another Rails application and it
works fine.
I have uninstalled and reinstalled all gems related to autotest, and
same problem.
I then deleted my ~/.autotest file so it's vanilla and that makes no
difference.
Finally, I installed autotest-growl to see what output is coming through
to that, and that works fine and I get the results of my passed / failed
tests, but the console for autotest continues to just pump out a string
of numbers instead of a description of the tests it is running.

I am using ruby 1.8.7

The following gems are installed:
autotest (4.2.9)
autotest-growl (0.2.3)
autotest-rails (4.1.0)

I have tried enabling and disabling redgreen, but to no avail.

Please can someone help me, I am absolutely stumped.

Matt