I have an Employee who is suffering from Bulimia & psychiatric

I have an Employee who is suffering from Bulimia & psychiatric problems. She has been suffering since December, we have sought treatment for her over the past 6 months. We have accommodated her problems at work giving support etc. She went off sick end of May, we had no contact with her for the last 5 weeks so wrote to her asking her to make contact to give us an update. She responded and a meeting was had to discuss her progress. Unfortunately she is very unfit to return to work, she produced a sick note for 4 weeks. Her mental state is very fragile and we cannot see her returning in the near future. We are currently paying SSP but we are a small Company (Childcare) and are having to employ someone else. We need advice as to if we can ask Employee to resign/leave of absence until she is fully fit. Prognosis is not good, she may be unwell for many months.
Many thanks
Ms Thomas