I think that is not the case... they used the Y chromosome. Not being a connoisseur, I have copied the survey's info for you from the Blood Of The Vikings site:

"The UCL genetics survey set out to discover if any genetic traces of the Vikings remained in the British Isles - and what this might reveal about the Viking Age. We hoped to find out where Vikings settled and roughly how significant those settlements were. DNA samples were taken from men at a number of sites. In the main, small towns were chosen and the men tested were required to be able to trace their male line back two generations in the same rural area - within 20 miles of the town chosen. The aim was to reduce the effects of later population movements, assuming that in between the Norman invasion of 1066 and the 20th century movement would have been limited.

The tests looked at the Y chromosome, which is only carried by men. This chromosome is particularly useful for population genetics studies as it is passed directly from father to son with virtually no alteration. Other chromosomes exist in pairs, one member of which is passed on from the mother and the other from the father. But because women do not carry a Y chromosome, geneticists can always be sure that this part of a man's DNA has come from his father, and from his grandfather before him. This chromosome allows geneticists to begin to unravel the male ancestry of the British Isles."

" if Mr and Mrs Viking had a daughter who then married a local the original viking mothers mitochondrial DNA would be passed on "

this is a curious point - if we except the Danelaw (where anglosaxon and viking intermingled, eg Yorvik) what strikes me is the amazing isolation that the ancient Britons seemed to suffer from (even until very recently). It's like two different countries in one. Wales has shown to be (quite obviously) the most 'native/non-saxon-viking' area (excepting the borders where possible numbers of Saxon settlers arrived through the centuries). And that's what's so fantastic in Ireland too. The Vikings were in Dublin as well, yet, it is quite possible 'each one to their own' was the motto, a bit like colonisers in the XIXth century. It is also usual for the vikings to take their own women and, being of an entirely different culture, they may have been happy to just trade, as DNA research shows. But I still wish there was more investigation into the legacy of viking culture and faith in the British Isles and other places. Striking too in that report how Scotland and Southern England contained the same amount of non-saxon (ie.ancient briton) genes. Which goes to show how wrong we are when we think population movements always involve extermination and/or mixing. After all, King Arthur was fighting an enemy (i.e. paradoxically, the 'English', ie. the Danelaw). And you don't usually marry the enemy's daughters, or if you do, it's in isolated circumstances. eg for political reasons.

My point is, when we go back in ancient history, there's so much that is so revealing (clearly so evident in isolated mountaineous areas for instance still today). Even though cultures have superimposed one another, a native Ur-ancient tradition must have remained in folklore and in our collective memory. That unseen tradition really interests me, hence the purpose of my other thread 'rub your arse'.