My previous manager placed my on health care leave and told me to take as long as I need to recover as my health is more important. But a new manager has taken over and has given me a deadline to return by the end of February even though I have now just been told I have lupus and am very symptomatic as I was originally being treated as having heart condition. I do want to return to work eventually but I have not even began my treatment to help control my lupus symptoms. Can he force me to return to work, I am no longer being paid and have even agreed to return doing 11 hours a week were as before I was full-time

You certainly cannot be forced to return to work, although the employer would be entitled to try to establish a reasonable timeframe for you to do so, OR to commence capability procedures if no reasonable timeframe could be established IF that is what they wish to to.

The employer should ideally discuss with you and take all available information into account - OH assessments, medical notes or maybe an independent medical review - in order to try to work with you to establish a suitable programme.

Of course if your condition is still being stabilised and it is simply not possible for a prognosis regarding suitability to return to work then they cannot go against this, hence why capability might eventually become an issue - you can of course still be dismissed whilst on long term sick, although hopefully this is not at the forefront of the employer's thoughts.

I have moved your post into a more appropriate forum where it should receive more attention

how long have you been absent for? I would say that if it is a considerable time it will be difficult to manage a return to work to a full-time position and so OH assessments and a phased return would be best but it may be that they have to consider dismissal on capability grounds. Are you in a company pension scheme as ill-health retirement may be an option if you are.