First stage - online written questions and answers.
Second stage - online tests, psychometric based questions.
Third stage - in personal interview in Aecom Aldgate office. Interview lasted 1.5-2 hours. Interviewed by two Aecom staff, both very friendly. Started off with questions about my past, education, what I was into, CV etc. Then general skills based questions, 'give us an example of when you've used this skill?'. Then showed me plans of a project they were working on and got me to point out key parts.