This doesn't tally with my experience CM, though of course it may just be a case of local ATC being super-helpful and sending DLAs without my intervention.
Correct. It's something we normally do without being asked if we know the aircraft is definitely going (lack of movement or lack of passengers around the aircraft being a pretty good indication), but on this one occasion it didn't happen due to being tied up with other things and was overlooked. Without that 'DLA' it will disappear.
Another thing to add to this is if you're departing from an airfield within the FIR, but will be joing CAS someway into the flight (examples - aircraft departing the Bristol area joining at Stafa or Monty; 20-30 minutes flight time away). It's often worth asking for a departure message to be sent if you're running close to the 30 minute deadline, especially in a slower piston engined aircraft as you may find that you get to the airways joining point to find the dreaded "sorry, got no details" message again