According to those Wiki figures:
Greater London [12.9%] + SE [13.7%] = 26.6% (as GavinC points out)
NW [11.2%] + NE [4.1%] + Yorkshire & Humberside [8.4%] = 23.7%
Therefore, the population distribution of London and the SE versus the 'Northern Powerhouse' catchment is 26.6% v 23.7%.
Not a particularly marked disparity by the numbers. Measured by capital investment in transport infrastructure from state funding, however, and I suspect those percentages will be vastly out of proportion.
Skipness ... I've never doubted that you do know your stuff, so could you clarify the point you were trying to convey in your 13:56 posting. Clearly you cannot believe that the North is sparsely populated?