Here's some information about WTC.
http://www.hmrc.gov.uk/taxcredits/tc956.pdf Fiz, when your reassessment comes around you might get PIP - catastrophic thinking such as "when I... don't get it" sounds rather like a depressed person.
That, combined with things you have said on another thread, has me thinking that you should not be applying for work unless it is perhaps one half-day per week on a voluntary basis.
You need to be eating a reasonable meal or two every day before you try to take on work even for a couple of days a week.
When I say reasonable meal, I do of course mean in a medical sense; what a doctor would recommend rather than what you currently think is OK. Your brain (not just the rest of your body) needs fuel if it's to work properly... fuel=energy=calories; you can't function properly on vitamin pills & less than 1000kcal a day.
Please try to shunt the ED out of your life as your first priority. >bighugs<