Over the next few days there were speculations about what that would mean if replicated at a General Election, with estimates of the number of seats likely to switch from Tory to LibDem put between 26 and 51. The statistician in me is wary of those extrapolations: there are lots of unknowns at by-elections, and British politics is especially turbulent at the moment.
On the other hand, political parties usually spend a long time building up profiles of voters. Lots of volunteers flooding in at the last minute is not a good substitute for that prolonged work, so there is more to the surge in Witney than simply the number of people who came to campaign. In fact, it will be far easier to win back people who voted LibDem in 2010, now they are able to see the difference between the Tories on their own and the Tories in coalition with LibDems.