Bear in mind that I haven't fully tested everything on my device, and my n900 may be subtly fscked up in a way I haven't yet noticed. Also please don't hate me if it hurts your n900.

It was basically:

Add a new repo in application manager. Copy of the Nokia one with 203 changed to mr0. I then disabled the original.

apt-get update
apt-get install mp-fremantle-generic-pr

It then downloaded a load of packages blah-blah-blah.

It craps out on hildon-desktop-applet-settings-mr0 (replacing 203). I got a dpkg-divert error.

To fix it (am sure of command only 90% certain that this is the file) I ran:

dpkg --purge hildon-desktop-applet-settings-203
dpkg-divert --remove /etc/hildon-desktop/home.plugins

I got the file for the dpkg-divert from the error message.

I then completed the upgrade with an apt-get -f install.

Like I said I have a few 203 packages remaining. With luck these will be upgraded next time!! Oh well what's the worse that can happen!