After a week or so frustration I was able to setup PPTP with freeradius authentication on Ubuntu 10.04, setting will also work in Ubuntu 8.04 but setting up freeradius in 8.04 is little complicated
I was also able to setup freeradius with OPEN-LDAP authentication with little more reseach and efforts
I documented the whole procedure her (without setting ldap with ldap though)
I will write the whole procedure here once I get chance.
Thank you and good luck with your setup