Days to build up Sominex? Or Amitriptyline? Promethazine doesn't build up. It's interesting on the whole 'doesn't work' point as for someone without chronic insomnia issues, I think it does (based on experience years ago and the number of people who are knocked out by Night Nurse) but now that I have serious, long-term sleep issues, it does very little. However, some people take a hefty dose and it works then.
Have you tried melatonin? It doesn't help me sleep through like it does for some people but it can speed up the onset and give me a few hours before the waking starts. It's great for some people though.
What about Nytol? It has the opposite effect on me but it works for my mam, who also has chronic insomnia.
There's also doxylamine who is another antihistamine. It's not available in the UK but you can order it from Biovea easily. A lot of people on here swear by it. It's the one thing I haven't tried.
I take zopiclone at times but I only get prescribed 14 a time to spread out and, now, my gp only continues to give me them as I have an actual neurologist-sleep specialist and he tells them they have to. This is not a long-term solution for correcting anything though.
I'm about to try trazadone as a nightly solution. Can't say anything about it until I do. Mirtazapine didn't work for me but it works for a lot of people.
One of the herbal things that not many people have heard of is magnolia bark. I've yet to try it as I have no luck with herbals usually but it's quite well documented that it can help sleep.