In a recent debate in Parliament a Home Office Minister gave the first official estimate of the benefits of immigration.
Joan Ryan said that “the Treasury estimates that migration has increased output by at least £4 billion and attributes 10 – 15% of economic trend growth to immigration.” 
What she failed to say is that immigration also adds to population to the same extent so the benefit in terms of GDP per head is roughly zero – that is there is no significant benefit to the host population. (see calculation below).
This is further confirmation of the statement by Professor Robert Rowthorn that “all the research suggests that the benefits (of large scale immigration) for the existing population as a whole are either close to zero or negative.” (Telegraph 2 July 2006)
£ 4 bn of out put on a GDP of approx 1250 is 0.32%
Net immigration in 2004 of 222, 000 on a population of 60 m is 0.37%
Trend growth is 2.5%. 15% of that growth is 0.375%
Thus on both measures quoted, the benefit per head is close to zero or negative. The major benefit goes to immigrants – which is why they come.
Migrationwatch are being kind in their assessment - the infleunce was given not as 15% but as 10 - 15%. 10% of trend growth is 0.25%, well under the immigration rate. I support immigration of people with needed skills. The duty of government is towrds citizens not towards immigrants. I would be interested to see how much of that increase is caused by the most skilled & highest earning 20% of immigrants. I suspect it might well be above the total for the whole. That is that unskilled non-English speaking immigrants positively reduce GNP.