Just that really - do you need to prove that not being granted flexibility would not be financially viable?
I'm in the position where, if I am not granted flexibility, my net salary would cover childcare costs but it would mean I wouldn't be able to cover all my current outgoings and I would be working such long hours that I wouldn't see my DS during the week. However my employer doesn't accept that there is a reasonable financial motivation behind my request and has said he thinks the issue is that I'm not ready to come back to work and to leave my DS (yes - he actually said this to me, along with the fact he had discussed my flexible working request with his wife in order to get a woman's perspective!!).