Create the following file, called 'yum.conf':

[trunk-ring3]
name = trunk-ring3
enabled = 1
baseurl = http://coltrane.uk.xensource.com/usr/groups/build/trunk-ring3/latest/binary-packages/RPMS/domain0/RPMS
gpgcheck = 0

[main]
retries = 20
diskspacecheck = 0
reposdir = /dev/null
syslog_device = 
assumeyes = 1
cachedir = /obj/yum_cache
obsoletes = 1
debuglevel = 1
gpgcheck = 1
logfile = /obj/yum.log
syslog_ident = mock

Now install reposync from yum-utils

Now run reposync -c yum.conf

and it should download the whole trunk-ring3 yum repository