AFC only specifies a working week of 37.5 hours - no mention of how these are made up.
However your current hours are normally deemed fixed by "custom and practice" so any change should be subject to consultation. And of course partnership working is central to AFC so staff should be involved at the planning stage for changes like this, not just being consulted after the fact. (No I don't know anywhere where this actually happens)
Look up your trusts "managing change" or similar policy. They normally start by saying how much staff are valued and how they should be involved in planning changes. Which is the bit HR always skip over.
At the end of the day management can always play the "needs of the service" card, so cover will be needed to be provided for the 12 hour days required. The only get out is that you should be able to arrange the cover amongst yourselves so that most people carry on 8:30 - 17:00 with a couple working 12:00 - 20:30 so long as the required service is still covered (look in your trusts flexible working policy under something like self or team rostering)
good luck
