Try this in your kickstart file:
%post --interpreter /bin/bash yes | yum update
Without the '--interpreter /bin/bash' the code will (I think) be interpreted as python. Also, when you run 'yum update' you are prompted to confirm that you want to go ahead and update the listed packages, and so you need to pipe 'yes' through it.
thanks for the response - with regard piping yes through to yum it seems you can just specify yum -y update and this will anser yes for you.
the interpreter bit is now causing the end of the kickstart to bomb out with the error of '/usr/local/bin/bash can not be run' as this is where bash is it seems. This is the %post bit of the kickstart
%post --nochroot --interpreter /usr/local/bin/bash /usr/bin/yum -y update
any ideas??