I agree with Tulula that it is less than perfect.
The risks of enforcement and time may be offset by the difference in the sale price and getting a buyer where there are none. If it is say £10k then that money you would put to one side in other investments so at least the property is sold ( rather than hard to sell or unsaleable at present) against the day there is a problem with the underlessee.
Of course the issue may only run as long as the planning consent is valid, and whether they might get permission in future.
You can offer to the freeholder to surrender the consent order, for a sum of money, to allow them to complete the works, if your flat, after completion of the extension, is saleable at a realistic re sale value.
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