Given the wording of your contract I think you will find it very difficult to make a valid challenge but there's no harm in asking, particularly if you have childcare arguments. Your chances would be greater if you were female (I guess from your name that you are male) but if you genuinely have childcare responsibilities you could as a long shot challenge the hours. To quote richtee's link:
Constructive dismissal can be complicated and difficult to prove so you should always seek advice before resigning. If your contract allows for a change, e.g. it reads, �you will usually cover the Midlands region but you may be required to work in other regions�, or �you will usually work 9 � 5 pm but may be required to work other hours as necessary for the business� you should seek legal advice. Even if the change being imposed by the employer is technically within the remit of the contract, it may be indirectly discriminatory on the grounds of sex & a breach of an employer�s implied duty to an employee of trust & confidence and a breach of the obligation to provide a safe working environment. In such cases, an employee may be entitled to resign & claim constructive dismissal.
It's a long shot though, and in the unlikely event you succeed it could damage the long term relationship with your employer.