I did this when I returned from maternity leave. Luckily the lady who had been filling in for me wanted to go part time also so we orgnaised a job share. She ended up leaving after about 3 months and I still remained part time. We hired an office junior fulltime to cover the rest.

I would propose part time or job share, not just part time. I think Nai has given some great advice above.

Good luck.
Janie xxx