Documentation[1] to the rescue, you just have to know where to look because
it isn't kernel specific...
Added this to dnf.conf:
installonlypkgs=kernel,kernel-devel
And now it's not removing kernel-devel... I'm not sure if the kernel one is
needed (not sure where the defaults come from and if they're overwritten
when you specify it yourself).
Question remains, this is the first time I've modified my settings so why
is my system behaving differently?
Thanks,
Richard