Paul, I've started from scratch and flashed my iMX6 with the MCE firmware from above (April 2014). Logged in OK.
I'll run through the steps, just to see if perhaps we've approached this differently, and hence me accessing the device via SSH, but you not being able to.
Flashed the firmware from the link above.
On first boot up, you get a series of set up screens.
On one of these, I had to enable SSH access, I think just FTP was the original selection.
After completing the set up procedure, and making sure the device was on my home network, I opened up an SSH client, and entered
192.168.1.104 (obviously this will be different on your device) as the IP address. I left the port as default (22).
I then entered root for the username and letmein for the password and click login (I'm using the Bivise SSH client - https://www.bitvise.com/ssh-client-download
- but I've not heard of filezilla being a problematic client before).
Connected fine, was able to ls etc.
At this point, I'm hoping it is just that initial setup screen where you didn't notice that SSH access had to be allowed.
If that is the case, it should be quite easy to fix. From XBMC's main screen, click on Programs, and then AMLinux Configuration. Click on Services, then make sure the Enable SSH radio button is blue. Return to XBMC's homescreen - I'm not sure, you may need to power down and start up your MCE iMX6 again before the setting takes hold. When in doubt... reboot
If you could follow the steps above, and let me know if you see anything different on your screen (in which case you probably have a different firmware in place on your MCE right now) we'll get to the bottom of this.