That's a very special piece of drive-by nastiness from the first poster, how very unhelpful to someone asking advice.
Choc, part of your strategy must involve talking to the personnel section of the company that mannages your contract - i don't know how they relate to your water company but you need to find out who manages you directly, and arrange a discussion with them. keep notes of this discussion.
I hope you have been to your doctor - if not get there pronto and discuss what's up with you and how it is affecting you. Even if doc does nothing, it is a matter of record then that you haveseen your doctor because of how ill you feel.
In an aggressive organisation, I am afraid that they are likely to terminate your contract - this is one of the reasons why certain companies prefer agency-type workers to employing staff, as staff they employ get more job protection. However you can only know this by talking with them as there is always the possibilty that they are sympathetic.
Ideally you also need to talk to someone from your trade union, if you are in one - if you aren't next time you are in work join a union, as it's times like this that their support pays off.
It's a rotten situation to be in - it's not your fault you are depressed - but bear up and think long term. If this company is not prepared to support workers thru difficulties then you are possibly better off taking time to get well again, then finding another job elsewhere.
Good luck