Hate to sound a book worm type but the new CAP 413 edition on the web is your best reference for this. Latest Edition just out 1st May 2006
Chapter 8 should give you all the details.Available on CAA Website.
1.4.3 If a pilot is already in communication with a civil or military ATSU, before the emergency arises, assistance should be requested from the controller on the frequency in use. In this case, any SSR code setting previously assigned by ATC (other than the Conspicuity Code 7000) should be retained until instructions are received to change the code setting.
Hope this helps.
