John Harvey is referring to the work of the Housing and Migration Network, which published a report on migrants in the private rented sector and will shortly publish a wider guide for housing professionals - see http://www.jrf.org.uk/work/workarea/housing-and-migration for more info.
What is known is that at least 75% of recent migrants (people who came in the last five years) are in the private sector. While some then start to get social housing and are recorded in CORE data quoted by Migration Watch, they have usually taken several more years to qualify and then be allocated a house. Remember that 'foreign nationals' can have lived here for many years, and that 'foreign born' people include those born abroad to British parents; and also that large proportions of recent migrants are - for different reasosn - not eligible for social housing anyway. For details on who is eligible, see the CIH website www.housing-rights.info