Is this a reasonable adjustment or not?

[TOWGA]

I’m autistic, recently started a new job, have told them I am autistic, and they did say about speaking to occupational health but nothing has happened yet,
I’m a person who likes routine and hates lots of changes, I do 3 days of 5 hours currently.
am I unreasonable to either ask for a set pattern or to ask for at least one set day off as a reasonable adjustment?

Sounds reasonable to me, and I would certainly accomodate that, but I don't know the nature of the work you do and whether that is feasible for them to accomodate, or to manage alongside other reasonable adjustments or flexible arrangements already in place for other members of staff.

I agree. It doesn't sound unreasonable but it depends.

If for example your job is explicitly to cover when other staff take a days leave, then it is less reasonable.

It’s retail, obviously I’m happy to be flexible and happy with notice to cover holiday etc. just at least would like one set day off a week if possible.