Here is a link:
http://news.bbc.co.uk/1/hi/business/2019385.stm
Obviously, it is a complicated issue, and it depends on how you do your calculatations, but it does say in the article:
'In the UK, Home Office research suggests that immigrants pay £2.5bn more in taxes than they take in benefits.
And the Treasury estimates that the economic growth rate has been boosted by a quarter-point because of immigration'.