I wouldn't fess up in advance. The first part is selling yourself to them. If they want to buy, then you negotiate. So if I were you I would:
- Research the job a bit. Find one or two interesting / relevant questions to ask that ideally show you off a bit. Phone them up and ask those questions. Sound enthusiastic, say you will be applying: prep them to be excited to get your application. Also, it might be worth asking about process. Most public sector jobs have a scoring system at each stage, so you also need to figure out how to frame your application so they score well. (I'd also research this independently - do you know anyone in the public sector that can help you get in the mind set, and review your application, if you are not in the public sector already?)
- Go to interview. Do all your usual interview prep. I would volunteer nothing about availability or T&Cs unless asked. BUT you need to prepare what you response will be if they ask you. Standard questions at first interview could be about your availability, pay rate and other T&Cs.
Again personally, I wouldn't say that I was on maternity leave and don't want to start until September (too negative). I would consider saying that something like 'I need to check, but I think my notice period is 3 months' it's April now. Likely they won't interview until early May earliest. Three months would be from an offer, so that puts you at September already. I maybe would consider saying that I was on maternity leave if it seemed relevant, I wouldn't necessarily though. Depends on the vibe in the room. It's none of their business. If I did volunteer it, though, I would something like 'I'm on maternity leave now, and looking to move my career on / get into this dynamic sector I'm v passionate about when I return.' I.e. direct them to your career focus.
If asked about package etc, be ready to include all your benefits (pension etc) I usually give these as a figure, rounded up a bit. I maybe would mention I was on four days a week, but not give a reason. (I've been on four days a week since before I had children. I used to do some freelance work in a different field on my fifth day, so when interviewing I explained that and included that in my ballpark earnings figure). If they asked about four days a week, I would be non-committal - might be looking for the same, might not, its negotiable.
- Have your interview rounds. Hopefully wow them and become their first choice candidate. Pick up info where you can about their working patterns. Maybe ask about flexible working in the 'any questions for us section' (they might volunteer this as a selling point to you - many places do. If they don't and seem v hostile, I might not raise it. But I'd also be pondering whether I wanted to work there.) Be ready at each round to ask the questions about your package and availability time scales. They will get more likely as things go on.
- Wait for the offer. Say thank you very much, you're v pleased etc. Ask for full details (if they offer you over the phone - you need an email breaking down the offer). Say you'd like to think about it, give them a timescale of a few days for when you will get back to them. They might tell you its urgent. Be firm. You need at least a day or two. bear minimum of overnight. Consider what you want. Consider how you can make it work. Go back and make that proposal, pitching not just what you want, but how you see it will work from what you have learned through the interviews. 4 days a week can be very attractive to them, they get almost all of their dream candidate at a discount. I would consider negotiating on pay. E.g. 90% of what they offer for 4 days a week. etc.
This can be done. I was interviewing while I was pregnant (big British FTSE firm). the process took ages. My now boss phoned me six weeks before my due date to say he'd sorted out the budget and wanted to hire me now. I asked to meet and explained at that meet that I was about to have a baby but was still v interested. We knocked around how it might work. I suggested 2-3 days a week from 3-4 months after (first baby, I was naïve). He raised his eyebrows and said: call me after the baby is here, lets talk then.
I did. We met when PFB was 6 weeks. He said, and I quote: 'I am definitely hiring. You are my top choice candidate. I know you just had a baby. Tell me what you need to make this work.'
He has been true to his word in both offer and execution. We negotiated around various things, including start date, pay rate and time commitment. We agreed 4 days a week with me going back at 6 months (this got pushed back slightly after I got an illness requiring hospitalisation when PFB was 4 months to allow me to recover). We discussed an earlier start date, I said I was only prepared to do that if I could do 3 day a week for 6-9 months. He said that didn't work for him. I started 4 days a week when PFB was 7 months (to the day!).
I hope some of this is helpful. Good luck.