Macbook pro (2011) failing to start
The problem was the keyboard. 

I detached all of the internal cables. The keyboard, the trackpad, the keyboard backlights, the optical drive, bluetooth and hard drive. 

I attached the HDD and a Keyboard via usb and started the machine by attaching the power cable. From there, success, it booted to desktop. 

I then started to reattach all until I found the culprit. Macbook now working minus the internal keyboard.