Fellows I’d like to share a useful source I just found. Here is the annual audit fees survey by Financial Director. Interesting part is they list auditors and fees for FTSE 100 and FTSE 250 companies, which solve the frequently asked “client list” question for Big four as well as mid-tier audit firms.
link to their latest 2009 survey is here: http://www.financialdirector.co.uk/digital_assets/1902/fd-audit-survey-09.pdf
The survey is a little bit dated but still provide a good start to research and update.
To check for any updates, simply search for “audit fees survey” on their webpage.