Is this a breach of employment contract?

You might want to re-read your contract AND your employee handbook. This is IF you actually have a contract. IF you company states you can only work 7 hours a week this does not mean you can come in early and leave early if they need coverage during specific core hours. Nothing states that the "core" hours cannot be part of the "flexi" hours, so before you go and lose your job, I would reconsider how you think your contract reads and how it is supposed to be read based on the needs of the company. You see a company CAN set specific hours within the "contracted" hours and make you work within those. The Key here is "managerial discretion" which you stated IS in the contract. Well it was up to management to require the new hours and unless you no longer like the job, you need to work with what they told you to do. This is NOT a breach of contract based on that one simple clause.