See Cap 393 The ANO
Section 2 Rules of the Air ( is all available for download at the CAA web site for FREE, use the following link
http://www.caa.co.uk/default.aspx?categoryid=33, click on search publications and search for CAP 393)
Look up Section 2 The Rules of the Air regulations 1996
Then see Section IV GENERAL FLIGHT RULES
Rules 5 and 6 which answer the Question.
I'm am not going to give you the answer because for you to ask the question suggests that you are not familiar with the details of this publication and if you are a active pilot then you should be.
have fun
Orions***