Sir William has been Government and International Affairs Adviser at Control Risks and Chair of the Ethics Committee since July 2012. He is also Director and Trustee of CRT Ltd and a Non-Executive Director of HSBC Bank Middle East. Since retiring from the UK Diplomatic Service in 2012, he has held a number of positions including UK Envoy for Aviation Security 2015/16. He is Director of WCP Consultants Ltd. He is also a Trustee of Turquoise Mountain Trust and Pact Global (UK).
Sir William joined the Diplomatic Service in 1975 and had successive postings in Abu Dhabi as Commercial Attaché, and then in Tripoli, Libya as Second Secretary Political. After a few years in London, where he worked successively on the Libya/Syria Desk and the Iran/Iraq Desk, he was posted to Australia as Head of the Political Section at the British High Commission in Canberra. Returning to London in 1992, he was appointed Deputy Head of United Nations Department. He then spent two years as an Overseas Inspector before being posted to Riyadh as Deputy Head of Mission and Consul General from 1995 to 1998.
On his return to London in 1998, Sir William was appointed Head of Middle East Department, where he served from 1999 to 2002. In 2002 he was appointed Ambassador to Sudan during the negotiations for a comprehensive peace agreement between the Government of Khartoum and the southern rebels (SPLA). In June 2005 he was appointed Ambassador to Iraq and served there for 14 months, during the drafting and passing in a referendum of the new Iraqi Constitution. He was Ambassador to Saudi Arabia from 2007-2010 and Ambassador to Afghanistan from 2010-2012. Sir William was awarded an Honorary Degree (LLD) from the University of Dundee in June 2016.