Ok. What you have is a basic lease based on the law society's business lease. Very rarely used by the profession due to its limitations.
The service charge is made up of the costs reasonably incurred by the landlord in performing the obligations in cl. C3 and in accordance with good estate management.
C3 sets out the services. Of relevance to the situation is the services of repair and deterioration of the common parts, structure, foundations, roof and exterior. The costs incurred must be reasonable.
A3.3 states you are to pay any shortfall between payments on account and the actual costs incurred.
A3.4 obligated the landlord to keep records, receipts, invoices. Quotations are not mentioned but it is totally reasonable to request to see any quotation for works to be undertaken.
If you consider the that the landlord is not acting reasonably then you can dispute the charge, you have a right to do so under cl. A3.5. Raise the dispute with the landlord first and allow 14 days for a response. If the landlord fails to respond or provide adequate information (do refer the the two guides I mention in previous posts for guidance) then you can refer the matter to a surveyor for determination.
The surveyor will be independent and act as an arbitrator. It may take guidance from the rics code but ultimately will be bound by the provisions in the lease. This means that the independent surveyor will determine if the decorating costs are reasonable.
There is no mention in the lease of how the independent surveyor's costs will be paid.
I hope this helps.