The sooner you serve the section 146 you can include in proceedings a claim for the amount certified by section 21. The certificate ( and for the sake of costs, copies of all the invoices) is included in the bundle that the lessee receives.
Is S146 the only or a practical response to the breach?
Based on the information posted, I offer my thoughts.Any action you then take is your liability. While commending individual effort, there is no substitute for a thorough review of documents and facts by paid for professional advisers.