Now I have the Fedora 7 installation working rather better I thought I'd see if I can still boot Slackware - I can!
It was no effort at all to get the Fedora installation to add Slackware to the boot menu, I just pointed it at the root partition on the drive where Slackware is installed (/boot is in the root partition) and that was it, it just worked! I'm impressed.
Now I can go and 'tune up' my Fedora 7 installation secure in the knowledge that if I screw it up really properly I can come back here to Slackware and sort things out (and do some real work too).