Advice for someone else wanted:
They requested a change in contracted hours to accommodate child contact arrangements (transfer of residency, meaning they can't put baby in childcare as they need time to adjust to new arrangements - they can only work when baby is with other parent for now, which was social services recommendation). They wrote a letter outlining their request.
The response was a quick discussion with personnel, who stated they couldn't reduce their hours (30 down to 14). No reason was given. Just that they couldn't do it. They also stated that the rota was done for the next 4 weeks anyway, and that nothing could be altered before then. A) rotas have never been done 4 weeks in advance before. The most is 2 weeks. Usually it's 1 week or less. B) they asked for next week's rota today, and it "isn't avaliable". Which essentially means it isn't done.
I've suggested they speak to their Union, but does anyone else have any useful advice on this matter? Fwiw the employer is a major supermarket. This person can work in all departments and was a manager until 3 months ago when they stepped down to reduce hours from 48 to 36 as a regular colleague.