I think it really depends on the nature of the job and how different the hours you want to work are compared to the hours they are advertising.
abovetheclouds is this the HR Admin job? If it's a 9-5 office job that's a bit different to a specific shift they need covered.
If you want to do 4 days, or full time hours compressed into 4 days or something, that's not very different. In that case I'd go to the interview and wow them, get them very interested in you, then approach the possibility of a small change then.
If you want to make a significant change to the hours, working 3 days or something, you are basically asking for a job share or for them to significantly rethink the job. In that case I think asking beforehand is the best way to go to avoid wasting everyone's time.
it also depends how crucial it is for you. If it's not a dealbreaker but would be a nice-to-have, then go to the interview and ask later. If the hours you want are an absolute must, then ask before.
I think in this climate they are likely to have lots of applicants so the incentive for them to consider alternatives to the hours they've specified is less. Working the hours they want for 6 months, getting good at the job, making your mark and learning all the ins and outs would make putting in a flexible working request at that stage more likely to be successful.