Where
13.—(a) the claimant has, on the advice of a doctor, refrained from work for at least 6 months immediately preceding the date of the examination on which the doctor's statement is based, and
(b)in the doctor's opinion, it will be necessary for the claimant to refrain from work for the foreseeable future,
instead of specifying a period, the doctor may, having regard to the circumstances of the particular case, enter the words “until further notice”.