Hi, I to have found a site called
www.workandbenefits.co.uk to be invaluable with help and info given when trying to fill in forms for any kinds of benefits. There is an initial £15 fee, but i found the info helped me to keep my extra benefit when being reviewed, so I considered it well worth it. It also gives info on what to expect and how to go about an 'appeal', and also completely outlines the Benefits Office 'points system' for claimants.
It tells you how everything works, very clearly.
I also have a son with mental illness, and have had to help with filling out forms in the past... the thing is, to ALWAYS write in your worst case scenario (how things are on your worst days).
I hope this helps. Good luck