I've worked it out...
Part 5 of the guide instructs to
Which appears to be a broken link. In my attempts to locate a download source elsewhere I ended up downloading postfix-2.3.3-2.1.el5_2.i386
.rpm and not postfix-2.3.3-2.1.el5_2.src
I've since downloaded the correct (.src.rpm) version of the file from http://vault.centos.org/5.2/updates/....el5_2.src.rpm
and have now been able to progress on to the next stage of the guide as the postfix.spec file is now present.
Thanks for pointing me in the right direction (literally letter for letter!) and hopefully this will assist others should they make the same silly mistake.