His basic thesis is that indeed we are not a nation of immigrants as many in the multicultural industry would have it, but a clearly defined, and until recently pretty homogenous lot.
As he points out, for Britain to be in reality a nation of immigrants the statement would have to be either philosophically empty (of course before the ice age there was nobody here at all therefore in that sense it is true) or disingenuous, as since just before the Norman invasion we have been pretty much ethnically stable.
Yes he will admit that we have been culturally more effected by ripples of immigration (Norman French, Jewish, Hugenot etc), but hardly touched ethnically.
The evidence he cites for his thesis largely comes from the new science of 'genetic archaeology' which has discovered through paternal DNA that a significant majority of those who live in the UK today are descended directly from the Ice Age hunters who first came here it seems from the Iberian peninsula. The figures are 88% of the Irish, 81% of the Welsh, 79% of the Cornish, 70% of the Scots and 68%of the remainder of England (over 2/3rds). (figures from 'The Origins of the British' by Stephen Oppenhiemer). So despite invasion s of Romans, Angles, Saxons, Jutes, Norse and whoever the ethnic makeup of the UK has hardly been altered.
Of course one of the side effects of this research is to massively undermine ideas of Celtic difference. It suggests that there is precious little such thing, if anything any difference predates and Celtic retreat from Anglo Saxons by 5,000 years.
Of course there is political import in these findings. The current wave of mass immigration created in part by our membership of the EU and our government's application of rules of access to all those from the former eastern block is and will continue to upset this stability. If when we think of Britain as a place which has been "for a long time, been both stable as well as liberal and tolerant, comparatively speaking", then this is in part because that nation was comprised of a people who were, by and large, homogenous. "Then, its ability to retain that character could well be under a far more severe threat from current levels of immigration than is made out by those who maintain that substantial immigration has always been a constant feature of Britain's demographic history from time immemorial".
In the conclusion he quotes Sir Arthur Bryant,
"The legal and spiritual association of men of different creeds, callings and classes in a nation, though often taken for granted, is a more wonderful miracle of cumulative human effort and wisdom than even the greatest achievement of science. For it enables millions who have never set eyes on one another to act together in peace and mutual trust. There can be no truer service than to preserve such a union, and prevent those millions from dissolving into antagonistic and destructive groups".Conway's essential fear is that if we as a nation continue with our current policies vis-a.vis immigration then we will cease to be a nation at all in any meaningful fashion, merely a geographical entity. Those who value the country should and must recognise this and set public policy accordingly.