Should I ring them up before the interview to discuss this?

Your lack of flexibility is a dealbreaker for them as they have specified it in the job description.

I suppose it depends on the job really to know what they’re likely to mean.
I work full time with two children and a flexible work pattern works better for me as it’s not a set 9-5.
I would give them a call to see if they mean a rolling rota, last minute shifts etc.

If it's an essential then they unlikely to compromise. That's why they've said 'essential.'

I'd save it for the day when they ask any other questions at the end.

I agree with this. And fwiw I think they could have defined it much more usefully than they have done!

Go for the interview. Partly cos if there's any flex their side they'll only show it if they like you enough. Partly cos an interview is about both parties working out whether they're a good fit.

I agree you should save it for the interview. You could raise it when they ask if you have any questions, ask what level of flexibility is required.
Most people going for part time jobs are working round something else like caring responsibilities, studying etc. So not many people going for the job will have total flexibility.

Good luck

Always better to save these conversations for after job offer, it's going to serve you no favours having it before, and they won't know how much they want you until you're interviewed, they'll need to toe the party line pre interview, but may be more open to negotiation post offer depending on the other candidates. Worst that can happen is you get the role and it doesn't fit your availability, but you've got good interview practice and internal gratification of success. It sounds open enough that you wouldn't be wasting anyone's time, flexible isn't easily defined!

Go to the interview and ask them what they mean when you have the opportunity to ask questions.

I would ask generally about the role in the interview but I would only specifically mention any flexibility constants if offered the role. The best time to get any leeway given would be when they’ve decided they want you.

They’ve put it in the role description to put off certain people from applying. People who have childcare commitments. The sort of people who are usually women.

I am sometimes a hiring manager in my work. I am happy for people to ask me questions about the job at any point, whether that is before they apply or once short-listed. You are wasting your time and theirs if it turns out the job needs and your needs don’t match. The needs of the job are not going to change no matter how great a candidate they think you are.
That said, the point above about interview practice is well-made, plus getting yourself known to them so I don’t think you lose anything by doing the interview and seeing what transpires.

Please don't wait until after the interview to ask the question.

I've interviewed for a role recently and was very clear in what was required in the job spec and the application form. 3 out of the 4 people couldn't do what I needed and thought it wouldn't matter and questioned if afterwards. Completely wasted my time and their time.

@holein1 well the difference is that the OP's job description is not overly clear

You stand a better chance of getting offered the job if you mention it after an offer; which is why people typically suggest that...

But if you couldn't/wouldn't take the job in its current guise; it seems a waste of everyone's time to take it that far. Flexibility seems to be important to them, as they've put it as an essential criteria.

I'd contact them and find out what they mean by flexibility and if it's something you could do or would want to do before I spent the time preparing/interviewing; but it's a personal call.

Personally, I wouldn't attend the interview.

Attend the interview, blow their socks off, then at the end ask them to elaborate on what they mean by ‘flexibility’.

I would get them to clarify upfront otherwise it could be a waste of time for both of you.

Good grief, don't waste their time.

Yes, I've always asked before interview so it's not a surprise to the panel. I was once told a role wouldn't be flexible but advised to remind them of my request when I interviewed. I got the job and we compromised on my flexible request because I had some wiggle room, which it sounds like you have too.

Absolutely not. You might not get the job anyway
do they interview see what they offer and then decide if it works for you.

Wait until the interview

Otherwise you'll talk yourself out of it

I'd appreciate the chat before the interview. We require a flexible attitude - it's a deal breaker, but that does mean we can't accommodate people's lives, it works both ways.

I would attend the interview and discuss it at the end regarding what they mean by flexible do t limit yourself in advance, as it's interview experience regardless!

I can appreciate your not wanting to do the prep if it's going to be a waste of time. That is of course your decision to make. Although any interview is good experience and practice so never a complete waste.

IMO, if you raise this before or even during the interview, I should imagine they will say they want complete flexibility as this will work best for them.

Having worked part time for most of my career, if it were me, I'd not say anything before and I wouldn't raise it at the interview either. I'd raise it once I was offered the job when they've already decided you're the best person for the job. It depends on the role though. If it's the kind of job lots of people could do well, that's unlikely to work. If it's a role that requires specific skills and experience where there will be fewer people to fit the bill, then I'd say worth a punt.

Obviously if you don't raise before and they raise it during the interview, you have to answer honestly.