"Immigrants are certainly not a drain on the economy. The evidence is that on balance they make a small positive contribution to the economy, with EU immigrants, because of their age profile, being ahead of non-EU immigrants (a group that includes people bringing spouses etc into the country who may be less likely to be working immediately)."
www.migrationobservatory.ox.ac.uk/resources/briefings/election-2015-briefing-fiscal-impacts-of-migration-to-the-uk/
You do realise that this study was a basic input output snapshot and did not take into account immigrants lack of previous contributions to the infrastructure they use (roads, schools, hospitals, housing etc paid for by our parents and grandparents), or the changing impact they are likely to have over time, dont you?
At present the demographic profile of the immigrant population is more youthful than that of the native British population, so more likely to be of working age, and more likly to be in good health, but of course two to three decades from now that will have changed,as their children will have 'taken' from the system, and their health declines with age, and they no longer work.
It is also distorted by the fact that it includes what one might call "high value" immigrants, doctors and many Australians, Americans etc working in highly skilled jobs in the city or in technology and paying higher rate tax. These high contributors compnsate for the majority of low contributors
The fact is that people who are only average wages i.e the majority will over a lifetime be net takers and not net contributers even when the snapshot has caught them at a stage in life of "contributing". This is going to be the situation even for 'hard working Poles' who spend their lives working full time in a distribution warehouse on minimum wage, as for a Somali migrant who has five children and works part time as a cleaner while living in social housing and claiming in work benefits, say no more!
The Migration Observatory you need to realise has a pro immigration agenda (its actually stuffed with immigrants) and is predisposed to present its finings in the most favourable possible light.