Thanks to everyone who replied.

It does appear that there aren't split files available for 1.5, and the 
install file deosn't explain about them, for understandable reasons.

I've also come to understand the mount command, though I didn't have to use 
it - just the option to install from an unmounted file system. The main 
problem I had was trying to edit the disklabel manually with ed which was 
solved when I realised I could label a partiton during the install. After 
labelling f the DOS partiton it was fine to install from unmounted file 
system wd0f.

Thanks
David