I am glad this is the newbies list ;)

The initial problem was 

# mkinitcpio -g initramfs-custom.img -k 4.5.0-eudyptula+
==> Starting build: 4.5.0-eudyptula+
  -> Running build hook: [base]
  -> Running build hook: [udev]
  -> Running build hook: [autodetect]
  -> Running build hook: [modconf]
  -> Running build hook: [block]
  -> Running build hook: [filesystems]
  -> Running build hook: [keyboard]
  -> Running build hook: [fsck]
==> WARNING: No modules were added to the image. This is probably not
what you want.
==> Creating gzip-compressed initcpio image: /boot/initramfs-custom.img
==> Image generation successful
#

The cause of the warning turned out to be that I had built the kernel
with an 'empty' config file. 

$ zcat /proc/config > config

instead of

$ zcat /proc/config > .config
 
(note the dot on config)

If I was more experienced I would have realised this when the compile
time was so short. It was only  while replying to Pablo with the steps I
had taken to build the kernel that I noticed my error.

So, thank you to the kernelnewbies mailing list.

Regards,
Tobin Harding