Thanks for your reply and apologies for the delayed response - I lost access to the forum while I rebuilt this machine.
Anyway... To cut a long story short I've reinstalled Centos 5 and reconfigured yum. The first thing I did post-install was to install yum-priorities, then configure the it with the following repos:
+ Centos: main, base, extras :: priority 1
+ CentOS-Media.repo :: priority 2
+ rpmforge :: priority 20
+ adobe-linux-i386 :: priority 30
I have installed all the good media stuff and all seems tickety-boo
Previously I had google and atrpms repos enabled. I *suspect* that atrpm was conflicting with rpmforge and or Centos-Media because the problems I had were largely with media based libraries.
I will be going back to atrpms for Mythtv packages later but when I do I'll be a bit more specific about what I'm picking up from each repo.
It's not terribly scientific but I think I can mark this one down as solved.
Incidentally the reason for the rebuild was because in trying to resolve any conflicts I removed all the packages not in my yum.conf file with a view to re-installing them later. Unfortunately that included all the stuff from the CD installation. That'll teach me to have a brilliant idea and not think hard about what I'm doing first...!