Barbara BB

My Mother is unable to sign TR8 on the Self Assessment return that she submits online each year.She is currently in hospital for a number of problems problems including Gout in her right hand which is preventing her from holding a pen to produce her usual signature but she is able to make a "mark".She is mentally competent according to the Doctors treating her.We do not have a power of attorney.What do we do ? Do we submit the form unsigned?