I have four slipped discs and am off work, i am thinking bout claiming working tax credit with the disability?

the criteria are stuff like can you walk without someone helping you etc etc, well i can do all that except one thing says 'if you cannot work 8 hours a day 5 days a week without being in continous or intermittent severe pain you are cled as disabled. well this is def me, but how do they go about checking and wat do they cl as 'severe' pain? is this pain bad enough or no? i am rather concerned as i think my employer is muttering bout medically dismissing me soon as this is the third month i have had off this year but i am hoping if i cut my hours to 16 that my back will cope with it as i love my job and do not want to be stuck at home alone all day. i am only 36