Hello!
Apparently CentOS installs, or will install, a really old version of iasl; here you'll see that they (still) use iasl-20090123-3.1.el6.x86_64: http://mirror.centos.org/centos/6.5/os/x86_64/Packages/ while the xgcc version is 20130626.
After doing some poking around it appears that IASL doesn't like the fact that my home directory is /home/andy.pont and I assume that it is the dot that is upsetting it as if I clone coreboot into a subdirectory under /home/andy then the default configuration builds correctly.
Thanks for your help and guidance, it is much appreciated.
Andy.