Can't set static ip

[renMike]

Software
XBian version: 1.0 (knockout) (kernel: Linux 3.18.8+)
XBMC/Kodi version:
Overclock settings: None

Hardware
Device type and model: Raspberry Pi Model B
Power supply rating: 1A
SD card size and make/type: 8GB
Network (Ethernet or wireless): Ethernet
Connected devices (TV, USB, network storage, ...): TV, wireless keyboard/mouse USB dongle

Problem description:
If I try to set a static ip for my pi running xbian + kodi from the System / Settings / XBian / System / Network - Type menu I find that I can enter the new address but when I try to save the new values I get the "Are you sure to apply modification?" message (press Yes) and then in the bottom right I get "Network An unexpected error occurred". The new static setting has reverted to DHCP.

From performing a web search I can find a similar thread but it doesn't seem to match my symptoms exactly.

Can anyone point me in the right direction so I can resolve this issue?

Mike