OK. I made some progress. Here's what I did in case it helps someone else.
perl -MCPAN -e shell
kept failing with 'not found db.h'
I tried all sorts of stuff to get Berkeley DB installed and nothing seemed to work but when I ran
yum install db
yum install db4
it kept saying nothing to do.
I tried to make Berkeley db manually but just got reams of errors.
After I got fed up, having tried a million things, I decided to untar the install files just to see what would happen...
tar xvfz ISPConfig*.tar.gz
And amazingly, Bob's yer uncle, it all worked fine and I now have ISPConfig up and running. I'm not sure which bit of what I did fixed it but there you go!
Thanks for your help,