Honesty is always the best policy. 

Your reasons for changing are probably the best I can imagine. If you don't say why you want a change, they may jump to less charitable conclusions.

If you already know what hours you will be expected to work, you could explain that you need a job which enables you to visit your sick Mum every evening/morning/whatever time you visit.

If I was interviewing you, I might ask follow up questions about whether there are any specific times of day that you cannot be available?  

You might want to ask why the post is being advertised as part-time, is it part of a job-share, or are they busy at a particular time of day? One of the most common reasons I used to advertise part time posts in my own NHS department was to fill in gaps in staffing levels due to school/child care timing issues. Many new parents are unable to return to work full time, so we'd work up job-sharing arrangements with people who preferred not to work between 10am and 3pm. 

Good luck with the interview
Dave   

Honesty is always the best policy. 

Your reasons for changing are probably the best I can imagine. If you don't say why you want a change, they may jump to less charitable conclusions.

If you already know what hours you will be expected to work, you could explain that you need a job which enables you to visit your sick Mum every evening/morning/whatever time you visit.

If I was interviewing you, I might ask follow up questions about whether there are any specific times of day that you cannot be available?  

You might want to ask why the post is being advertised as part-time, is it part of a job-share, or are they busy at a particular time of day? One of the most common reasons I used to advertise part time posts in my own NHS department was to fill in gaps in staffing levels due to school/child care timing issues. Many new parents are unable to return to work full time, so we'd work up job-sharing arrangements with people who preferred not to work between 10am and 3pm. 

Good luck with the interview
Dave   

Thank you, I've always believed honesty is the best policy. And my current employer has been less than supportive at times which has pushed me to look else where. 

How do I approach the subject if the question leading to that discussion doesn't arise? 

Most interviews end with the question "do you have any questions for us?, I'd drop it in there if they haven't already covered it.