Ridsdale and Simson were appointed joint CEOs in 2014 following the departure of chief executive John West. The duo were previously directors at the business, joining in 2005 and 2010 respectively.
Recent client work for the London-based Tavistock include advising Dee Valley Water PLC on its recent £84m takeover by Severn Trent, and Sirius Minerals’ $1.2bn project fundraising to start UK’s largest mine development in a generation. Other clients include Hammerson and Standard Life’s Brent Cross Cricklewood Development Project.
Simson said: "We have achieved a great deal in the last two-and-a-half years and look forward to building on this momentum. We have won high-profile new clients, strengthened our team, enhanced our digital offering and put in place an extensive global network of over 30 partners. We will continue to build on our areas of strength and add to our high-quality team of advisers.
"Matt has played a big part in building Tavistock over the last eleven years. I'm delighted for him that he is taking on this exciting opportunity with Camelot and the National Lottery."
In his new role, Ridsdale will join the Camelot UK executive board and report to CEO Andy Duncan.
Ridsdale said: "Tavistock is an outstanding firm built on a great team. I have enjoyed my time there enormously and now look forward to joining up with the team at Camelot next month."
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