Declaring emergencies
Declare the emergency on the frequency you are already on whether inside or outside controlled airspace. The controller should then pass your details to D+D and seek assistance from them as appropriate - this may mean you are then transferred to 121.5 or, as in most cases, "operational control" of the emergency will be granted to the agency you are working, although D+D will still have "executive control" of the emergency. If for some reason you can't get a word in on the frequency you're working, chop to 121.5 and declare the emergency on that but remember to tell D+D who you were working prior to the call and that agency doesn't know you've left their frequency. The point above about the emergency SSR is very relevant because, as soon as it's on, D+D will be chasing down who is/was working the aircraft wh.