John - you need to make a distinction between population growth (ONS figures) and the household growth (DCLG figures). Both use the same starting point, so the difference in the ratios is based on demographics. This means that this projected number of immigrants will typically live in larger households, and/or will have larger numbers of dependent children who will not as yet have formed independent households of their own.
This demographic trend also has relevance to the size and type of affordable housing being built - if the predominant population growth in the next 25 years will be from immigration, and those people will predominantly need accommodation to suit larger households not smaller, then the current trend to deliver smaller affordable homes to suit smaller households is misdirected.
This was pointed out in a report by Policy Exchange; "Housing People; Financing Housing", who said:
"The current situation in relation to overcrowding will get worse before it gets better, particularly for social housing tenants... in recent years, the wrong type of social housing has been built. The dominant need within social housing is for larger bedroom houses, yet the number of three and four bedroom properties being built has declined significantly. The rationale for the smaller unit building programme is the anticipated demographic change to more single households. However, this is a flawed policy assessment".
Your assertion "they cannot be affecting the current growth in waiting lists if they are (mainly) ineligible to go on them" is incorrect.
If all the available private rented accommodation is taken up by incoming migrants, local people find it difficult to obtain private rented accommodation and more apply to the local authority for social housing. There is a direct link.
I have no doubt that Migration Watch, being a pressure group, convey a biased story to support their argument. You have done the same in the opposite direction. If I argue against someone else's view I tend to do the same. It is how debate works.
However, when it comes to impartial analysis of figures, it is fair to say that having a population rise 2:1 driven by immigration is not going to help alleviate the housing problem in this country, is it?