Provided that the commander has taken note of the circumstances of other crew members and is satisfied that the flight can be made safely, then he may at (his) discretion extend the FDP beyond the tabled limits up to a maximum of 2 hours for an FDP involving 2 or more sectors and prior to the first and subsequent sectors. However this may be extended to 3 hours prior to a single sector flight or immediately prior to the last sector of a multi sector flight. 3 hours is the maximum permitted unless a situation arises which constitutes a defined "emergency" when the 3 hour limit may also be exceeded.
The original extension is calculated according to what actually happens rather than what was planned to happen.
As such in the example you present the flight was planned as 11:45 and actually resulted as 12:12 therefore the Captain may after taking into account the previously mentioned criteria exercise discretion as (he ) sees fit.
The reference to "unforeseen circumstances" relates to the extension of duty after a reduced rest period, when such application can only be done exceptionally, and then only to the extent necessary to allow for unforeseen circumstances that become apparent during the last sector.
In your example there is no mention of previously reduced rest being a consideration, therefore the flight time being longer than scheduled is irrelevant for the purpose of "unforseen circumstances". The commander may therefore excercise discretion as (he) sees fit. In fact it is an easy situation.