This is a good question to ask about any company, and a great question for the industry as a whole...
Currently the answer is no one! Unless our work undergoes third party verification. (Which we have a fantastic track record of!) This is why we are so transparent with our work and try and take the black box out of sustainability.
We currently see this lack of global standards as a real problem. We are part of a group lobbying UK government to improve policy around it.
We believe that everything we do, in terms of standards and methodlogy, is beyond the industry norm at the moment. But we are always keen to discuss specific concerns or standards that matter to our clients.