"Your contractual hours of duty are ten hours per week as determined by your Manager from time to time according to operational requirements. Yours hours of work will fall between the following time bands although these may vary from time to time according to business needs .... your contracted hours are part of the terms and conditions of your employment and it is a requirement of the post that you regularly achieve base performance ..... in addition to your basic hours you will have the opportunity to earn additional money in accordance with the company's PBR Scheme. This voluntary arrangement which in no way affects other terms and conditions of your employment from time to time the Company may exercise its discretion to alter the terms of this scheme subject to a month's notice."