Here on the North Sea (& allowed by JAR) we have an en-route letdown procedure. I don't have the books in front of me but I am fairly sure the requirements are: at least 10 nm from land (as determined by wx radar), an expected weather of at least 600' cloudbase and 4km vis. If we are not clear of cloud by 500' we must give up and go IFR.
By night its 1200' and 5km for reasons I have never understood! ( - what's the difference between day and night when you're in a cloud?).
If I have got the figures wrong, no doubt some other N Sea chaps will let me know
