How important is it to you to work these hours?
Officially if your hours were specified in your contract, as oppose to mine which just states the total number per week, then you needed to have had it changed for the new ones to be agreed.
However, if you have been doing the stated hours for 5 years it is likley to be assumed contractual now, but its not cut and dry.
If you have a good reason to work them, then I suggest you submit a flexible working request, asking to work them, say why, and remind them that it has worked perfectly for 5 years.
If they refuse then you could take it to your union or get some legal advice to help take it further.
Seems suchme to have to go through all this for 15 minutes. Why can they not accommodate it any longer?