If your tenant has served a break notice on you your first check should be to see if there are any break conditions contained in the lease. If there are you should note that there is no legal obligation on a landlord to assist its tenant in complying with those conditions. It is up to the tenant to comply with them, if they want the break to succeed.
If the break has particularly onerous conditions, such as the requirement for a tenant to "perform and observe the covenants contained in the Lease", you should be aware that serving a schedule of dilapidations may be taken as communicating what the tenant needs to do in order to reasonably perform its repairing, decorating and yielding up obligations. You may therefore want to refrain from serving the schedule until after the lease has terminated (subject always to the other terms of the lease).
If you do want to serve a schedule of dilapidations (with a view to reaching a monetary deal or to require various alterations to remain) then it should be served on a without prejudice basis to try to avoid your tenant arguing you have limited the action the tenant must take to comply with the break conditions.
However, if the lease requires you to serve notices to oblige reinstatement, then those notices should be served on an open basis.