A spokesperson for the Greater London Authority said:
"The GLA have employed an executive search firm to search for individuals to fill two to three senior positions. These essential roles will support the Mayor in delivering his vision for London and it is therefore crucial that highest quality of applicants are put forward for these positions. Executive search firms have the capacity, network of contacts in the specialist fields and the ability to approach a range of individuals about the roles in a confidential manner. It is expected that this process will enable the recruitment of exceptional individuals with track records of success in their fields of expertise. The cost quoted is the maximum payable, in practice, we expect the final costs to be less."
This follows my observation here.