The terms of your contract of employment, including your working hours, are ultimately determined by the realities of what happens in your day to day work. Therefore, regardless of what your line manager has relayed to HR, or what is set out in the contractual document, your employer should accept that your working hours are the pattern you have followed for the past two years. Even if your line manager had never formally agreed your working pattern, the employer would have been deemed to have accepted it as a term of your contract by the custom and practice of you working those hours each week for the past two years.
If your employer accepts that point, they cannot then vary your working hours without your agreement. Any attempt to impose the variation would amount to a breach of contract and you could leave and claim constructive dismissal.
In order to try and secure your agreement to a variation, your employer would be expected to engage in consultation with you. This consultation process may be very similar to the redundancy situation that you recently faced in that your employer would hold meetings with you to explain their position and explore possible alternatives to their proposal. They would also need to address the points and concerns that you have already raised about how your working hours could fit with their new proposals. Whilst your employer does not have to agree with you, they would be expected to justify why they need to make their proposed changes.For example, if they have previously accepted two people being available to cover the role at any one time, why have they now decided that there must be three people at all times?
The aim would be that even if you could not agree to their original proposal, it might be that you could come to a workable solution and agree a variation to your contract on those terms.
However, employers are entitled to manage or restructure their business as they see fit, provided they do so for sound business reasons and they act reasonably towards the affected employees. This means that if, after appropriate consultation, you are not persuaded to agree to a variation, one option open to your employer is to serve notice to bring your employment to an end and offer you a new contract with the revised working hours. If you do not accept the new contract you will have been dismissed and your employer would have to show that this was for a fair reason. The reason your employer would most likely give is that it is for “some other substantial reason”, which is a potentially fair reason for dismissal and can include circumstances such as a business restructures and reorganisations like this one.
If your employer does not offer any justification for their decision, but then still impose the change or dismiss you, then you may be able to pursue a claim for unfair dismissal. Also, you might try and argue that their decision would impact more on women than men because women have the majority of the childcare and therefore it is indirect sex discrimination. However, your employer is likely to be able to argue that they can justify this change for good business reasons.
Before matters come to that point, it is in the interests of both parties to be as frank and open as possible with each other during the consultation process. Given that you are an existing employee, you know the business and you know the role, it would generally be in the interests of your employer to retain you and avoid having to recruit someone new if you were forced to leave. Therefore, I would advise that you explain the difficulties that you do face with their new proposals, but also the fact that you are prepared to be flexible as far as you can be. Hopefully, this will allow you to come to a mutually beneficial agreement to the situation.