There is no standard form for this. The communication must be ‘in writing’ (see clause 13.1) and must make it clear what it is.
Please note that this is a two-part process. First the employer needs to notify the contractor that they wish to terminate, and then they issue a certificate if the reason given complies with the contract. You must also bear in mind with reasons 2 and 3 you must first give a notification of the default and then allow the contractor 2 weeks to stop defaulting before you can notify your intention to terminate.