http://www.computerworlduk.com/toolbox/open-source/blogs/index.cfm?entryid=2...
Note section 5.2.1 of linked PDF document:
"However, public administrations may decide to use less open specifications, especially in cases where open specifications do not meet the functional interoperability needs or the ones available are not mature and/or sufficiently supported by the market, or where all cooperating organisations already use or agree to use the same technologies."