OK, a brief update for installing on lenny and upgrading afterwards:
Had to change a few packages from the lenny package list and had to use the gentoo apache fcgid config to get it to run as fastcgi isn't available with lenny.
In brief (I will create a proper list if putty has logged everything later)
Updated the package list (proftpd-mysql -> proftpd, libapache2-mod-fastcgi -> libapache2-mod-fcgid, libsasl2 -> libsasl2-2)
Did a debian install as normal
Updated apache config for fcgid calls from the gentoo config
Restarted apache, checked ispcp worked and could login
Copied /etc/vhcs2/vhcs2.conf to the new server /etc/vhcs2/vhcs2.conf
Copied the vhcs2 db over to the new server
Ran the update script
It looks like it's done what I needed, I just need to move all the user files over to the new server