Down has been group finance director since 2008 at Hyder Consulting, which was recently acquired by super-consultant Arcadis.
Mark Rogerson, Chief Executive Officer, said: “We are delighted that Russell is joining us. He has a strong financial and commercial track record and in depth experience of working in a PLC environment.
“Speedy will benefit from his highly analytical and focussed approach and I very much look forward to working with him.”