Hi cockerdoo,
Just a few days ago we posted the exact same problem about roxy our cocker pup. We were close to handing her back as we couldnt take it anymore.
We got some good advice like you are getting on here and day by day she is getting better. We have got her downstairs now with minimal whimpering and then shevsettlesed. She has got used to having her kong when its bedtime, it really helped take her mind off us leaving the room.
Another thing that really helped although seems simple is to tire her out loads with training, she gets visibly mentally tired and curls up on the floor when werevwatching tv, we let her nap a bit but then keep her going til its our bedtime, she has now started to walk off to her cage in the other room to get some rest!
She cries at about 4am still but its not the howl it used to be. I get up, put her outside and stand with her, she wees and then i put her back in her cage and give her a treat and go to bed, never saying a word to her to keep her as calm as possible.
Keep up and unwill find it gets better after it gets worse, thats what we found anyway.