cancer of the womb

Hello i have had cancer of the womb and had to have a full hysterectomy  i Have been signed of work and my employess keep contacting  me to carry out work duties while i am off work sick  .   Are they  allowed to do this   .  I have asked them to stop contacting me as iam not able to carry out work duites during my sickness.  They still keep bothering me  and what me to work  while i have a certified sick note and they no  my condition.