With effect from 6 April 2010 a new form for doctors to certify a persons fitness for work will be issued. It will add to the existing fit/unfit statement the following

If available, and with your employer’s agreement, you may benefit from:
a) a phased return to work
b) altered hours
c) amended duties
d) workplace adaptations
e) Comments, including functional effects of your condition(s):

This will enable employers to determine whether they can change an employee’s work pattern so that limited or certain duties can recommence.

Look at the details on the DirectGov website