Immigrants Taking Over Our Jobs?

This line has been thrown up so many times that it has become a political vote winner latched on by all the major political parties in this country to court for our votes.

Last week we woke up to the news that the 10400 jobs up for grabs at the Olympics could not the filled by G4S, the security company contracted to employ people to fill this vacancy. They had to draft in Soldiers from their annual leave to fill the void.

The question running through my mind since the news broke is where are the almost 3 million people on the dole queue whose jobs were supposedly taken by Immigrants?

If these 3 million unemployed are actually looking for employment, shouldn't they be scrambling for the 10400 vacancy?

Can somebody please tell me why this vacancy could not be filled in a country with 3 million unemployed?