It is usually a mistake to type by-election vote shares into election predictors, but what we can do is to look at the tactical voting effects which we have observed here.

When UKIP was not in contention for the seat, over half of its supporters defected. But when the Conservatives were seen as outsiders only about a tenth of their natural supporters defected to vote tactically.

This is potentially very important. In many seats, UKIP is not going to come first or second, so it matters if UKIP supporters are going to vote tactically and support the Conservatives.

We can use these tactical voting (TV) fractions in the Electoral Calculus prediction model, whilst still using the national opinion polls as the base level of currency party support.