The only problem being is that was JAR FCL1 v4 that sleepybuha is referring to and was suprceded, the current version is Amendment 7 and that refers to conditions not rules. The UK CAA have currently adopted Amendment 5 which is what our ANO refers to. However the section remains the same between the 2 amendments.
http://www.jaa.nl/publications/jars/607069.pdf
Page 1-A-12 refers.....
* A pilot may log as instrument flight
time only that time during which he
operates the aircraft solely by reference
to instruments, under actual or
simulated instrument flight conditions.
(5) Operational conditions:
(i) Night
(ii) IFR