My bad, the original planning conditions for Wembley were 50k capacity for extra events (they must have a calendar/a fixed number of days-a-year for 'full capacity' events).
In March the local planners gave them permission to use the full 90k capacity for all their PL games, so it looks like we could get a fairly hefty allocation on Boxing Day.
http://www.skysports.com/football/news/11675/10811931/tottenham-secure-approval-to-play-wembley-matches-at-90000-capacity