In the last three elections, there were some trends.
In 2015, 2 weeks before the poll, Labour was averaging around 35. A week before the election, they were averaging around 33. This is a difference of two points. The final result was 31, a further shift of two points.
In 2010, 2 weeks before the poll, the Tories were averaging around 33. A week before the election, they were averaging around 35. This is a difference of two points. The final result was 37, a further shift of two points.
Again, 2005. Labour, 40, to 38, to 36.
In 2001, Labour went from 49 to 45 (4 points) then to 41
In the past week, Labour has moved from 34 to 38. If this trend were to continue, Labour would end this election at 42%.
One problem with this trend system is that while it does work for the parties outlined, in many cases, it is way off. In 2010 the LibDems did not changed their trend very far but lost a large number of points on election day.
Regardless, this is today's prediction.