After not understanding the reason why I couldn't perform the upgrade immediately to 6 from my version of 5 (getting slapadd failed errors) I thought I would try again on 6.0.3.

So trying 6.0.3 it also failed with the slapadd import failed and decided this time I better check logs and discovered in /tmp/zmsetup.log

Code:
Thu Nov 26 10:38:09 2009 *** Running as zimbra user: /opt/zimbra/openldap/sbin/slapadd -q -b '' -F /opt/zimbra/data/ldap/config -l /opt/zimbra/data/ldap/ldap.60
<= str2entry: str2ad(ZIMBRADOMAINPREFMAILSIGNATURE): attribute type undefined
slapadd: could not parse entry (line=1142)
Thu Nov 26 10:38:09 2009 slapadd import failed.
Doing a search on "ZIMBRADOMAINPREFMAILSIGNATURE" I discovered that this was part of the domain disclaimer extension I had installed and uninstalled donkeys ago as it really wasn't what we wanted and upgrading to different versions with it installed got messy. However this attribute had obviously remained in my LDAP db somehow even though it was not in my schema.

So following this link on exporting/importing my LDAP data Exporting and Importing Zimbra LDAP Data » Zimbra :: Blog I removed these attributes from the dump

Code:
ZIMBRADOMAINPREFMAILSIGNATURE:: 
ZIMBRADOMAINPREFMAILSIGNATUREHTML:: 
ZIMBRADOMAINPREFMAILSIGNATUREENABLED: FALSE
Reimported my LDAP data and hey presto the upgrade went like a dream!!!

Hopefully others who have installed the domain disclaimer, removed it and then attempted the upgrade forgetting it makes changes to your schema will benefit from my post