You are correct to say that there are two ways for the service manager to require the contractor to carry out works in addition to the routine work required by the service information. However, service manager’s instructions are not limited to permanent changes in the service information as you suggest. The only limit is that the service manager cannot add a different service not covered by the definition of the service in contract data part one.
So, in your example, the service manager can just instruct the contractor to clean block C, either from now onwards or as a one-off job. Equally, if the contract requires blocks A and B to be cleaned once a week, the service manager can instruct that block A is to cleaned twice a week for a couple of weeks. All of these are changes to the service information in the sense that they add something. All can be then be dealt with as compensation events (see clause 60.1(1)), which can be valued by the rate in the price list for such cleaning (see clause 63.1) or, if there is no rate, by calculating the effect on the defined cost plus the fee (see clause 63.2).
Option X19 is designed for more complex project-type work, for example build a new block D. For such work you would require a detailed programme and price before it starts. Option X19 is not intended for every-day, run-of-the-mill reactive maintenance that is needed to operate and maintain the asset that is the affected property