Well, this problem is usually related to the back of the ignition lock (ignition switch), not the entire ignition lock assembly. Look at the linked image. I refer to the part where you can read "VA" upside down.
This image is from the post mentioned before by fellow rijosho (http://www.defendersource.com/forum/...6&postcount=56
In case you need the part numbers (for 2007> years):
QRF500090 - LOCK ASSEMBLY-STEERING COLUMN
YXB500170 - SWITCH - IGNITION
For models from 1987-2006, part number vary...
QRF100880 - LOCK ASSEMBLY-STEERING COLUMN
PRC2734 - SWITCH, WITHOUT STEERING COLUMN LOCK, STARTER, WITH DIESEL ENGINES
PRC8230 - SWITCH, STARTER, 2.5L 4 CYL TDI L/R
You should check http://lrparts.net.ru/
(online microcat) to be sure.