I returned to work in January having agreed part time hours. I am now being asked to change my days as another part time worker is due to return after her maternity leave in July. My contract says I may need to work flexible hours, but I will find problems finding childcare on alternative days.
From your question, it sounds like your employer agreed to your request to change from full time to part time working in January 2009. If an employer agrees to a change in an employee’s terms and conditions, this becomes a permanent change in the employment contract going forward.
Legally therefore, an existing employment contract can only be varied with the agreement of both parties. So if your employer wants to alter your employment contract, he will have to formally consult with you and try to get you to agree to the changes. If he cannot, then your employer would have to terminate your employment and rehire you on new employment terms. If they went down this route, your employer would need to follow the new dismissal procedures which came into law this month and failure to do this might give you a claim of unfair dismissal against them.
In regard to the clause in your contract stating that you are required to be flexible, you might want to talk to an employment lawyer to advise you further on this clause but just from a literal reading of it, I would take this to mean it allows your employer to change your hours or working arrangements on an occasional, ad hoc basis perhaps if there was a particular project on the go. This is not the same as trying to get you to agree to permanently change your days of working.
Ideally, you want to try and resolve this amicably and I would suggest that you have another discussion with your boss, explain how you and your colleague catch up with work from Monday on the Tuesday and you can’t see any reason why this would not continue to be a satisfactory arrangement. Also remind them that they were the ones who suggested back in January that you should not work on Mondays.