Ok, so you are entitled to 30 hours over 4 days, starting at 7.30. If business reasons mean this isn't available then the same role starting at 8.00 sounds reasonable.
You can refuse 8-6 that's fine.
I take it you are the one who wants to reduce your hours to 24? Did you make a proper flexible working request for this? They have to consider the request but don't have to agree it as long as they give appropriate business reasons for the refusal.
It sounds as though you asked for 24 hours over 3 days, and they offered you a compromise of those hours over 4 days, presumably because they need a certain amount of cover by senior staff, or an alternative option of fewer days in an alternative, more junior role. Now the option of reducing your days is no longer possible. Have I understood that correctly?
It sounds as though there was a bit of a mess up if they offered you a compromise that wasn't available, but if it isn't it isn't.
Your rights here are to your original role on same or no less favourable terms and conditions, or if you want to reduce hours, you have a right for them to consider it and follow an appropriate procedure for that.
In your position I'd go for 24 hours over 4 days on your original role as that sounds like a good compromise to me. Did they follow a flexible working procedure, assuming you made a formal request?