I have been swapping emails with the Buffalo tech support team since posting here and have an update!
As I had access to the data it was recommended that the next thing I could try would be to remove all data and copy it elsewhere and force a firmware update, delete the user-config files AND delete the partition.
Took me about a day or so to copy the 250GB and 10's of thousands of Music files and all my photographs to other storage and this afternoon I crossed my fingers and went for it.
After the expected clicks, beeps and flashes I attempted to login to the japanese login page....
SUCCESS! I was in! I then followed instructions on here to change the language back to English!
All is looking ok again now. I can find the NAS on the network and access it still.
I'm just in the process of transferring a load of stuff back.
I'm glad all is working again now.
Thanks to everyone for their suggestions! Much appreciated!!
Simon