You are under no obligation to discuss your diagnosis, it's confidential to you, so handle it as you see fit.
It might be an an idea to look at employment advice, others have reported Macmillan are good with this.
Can you just say you are seeking a better work/life balance and leave it vague? There are lots of reasons to want to change to part-time work (I did so to study for example). Thinking about it, they may not really be allowed to ask in case it is construed as trying to find out if you have childcare commitments and therefore could constitute sex discrimination.
Good luck in your interview!