Trafford,
You should just leave work. No job is worth that. I have been in jobs before, where you had to work very hard, and see other people (in the easier sections) taking full lunch breaks & no stress. I can understand how being in such a job can affect your whole life. Even when you are not at work, you are probably thinking of all the work you have to do, not to mention the problems with colleagues fighting.
Another poster said to try and get another job. This is easier said than done. It could take 6 months or a year to get another job. More damage could be done to your health by waiting for another job. Also, if you are under such stress/pressure, it may show in interviews and you may be less likely to get another job.
I don't think widening your search/taking a pay cut is the answer to your problems. Sometimes, you could take a pay cut and find that you still have a lot of pressure/can't fit the work into the 40 hrs. Also, prospective employers often want to know WHY you are taking a pay cut. Telling them about the problems with this job is not an option, as (in my experience) prospective employers have no understanding about stressful workloads, long hours, bullying and periods of unemployment.
You could ask for 2 wks hols, and use this to relax, set up some interviews etc. Or else you could leave, and go travelling for a few weeks. Then when you return, you could tell prospective employers/agencies that you left your last job to go travelling, and that you are back now and looking for work. In my experience, it is often easier to get work when you have no job, as you are available for an immediate start.
I would not really recommend going on sick leave unless you are very sick, as it is not nice to have this on record for future jobs.
It is all very well complaining to HR about all this, keeping notes on incidents etc, but bringing a case could be a long process. There are no guarantees of being successful, and all this would just be prolonging the agony.
Best of luck,