Is there not a "sleep clinic" run in your locality? I remember (about 25 years ago) when my daughter was little and would never sleep on her own, in desperation we went to the clinic. With their advice it was sorted within a week.

Sorry it was such a long time ago I can't remember exactly what the advice was....BUT it was something along the lines of keep going into the room to reassure the child, make them comfortable but being assertive to ensure they know its time to sleep. Also activities an hour before bedtime, which MUST be at the same time every night, should be quiet like reading a book together. Also you need blackoutcurtains in the room so they know it's night time.

Anyway, on keeping going into the room where they are sleeping you gradually increase the time you allow them to cry before going in....that's all I can remember so hope it helps. You have my sympathy as I still remember what it's like having no sleep yourself as a result