Survey: Mosques in England and Wales
BBC News, Survey reveals mosque insights, 23 Feb 09
"The average mosque in England and Wales attracts 400 attendees at Friday prayer meetings, while providing a range of community services, a survey has found."
The report is detailed here:
Charity Commission, Charity Commission publishes first detailed mosque survey
Charity Commission, Survey of mosques in England and Wales (PDF). It's 38 pages. Haven't read it yet.