The issue is embroiled in politics at the moment, in that Roy Vandermeer QC's report is expected to recommend approval of the new terminal. This report is due out shortly, However, government approval is not expected to come until after the election in the UK - which should be in March or April. I don't think there's much political in that, given that everyone expects approval.
The questions surround the issue of limits on flights and whether the government will accept recommendations against a third runway. The growth of air traffic means that it will double in the UK over a period of 15-20 years and there is a serious need for a new runway. This is expected to be at LGW, but it should also be hoped that Northolt can be expanded and its proximity to LHR may allow it to become a kind of satellite airport, with quick, direct links.
We'll see, but nothing is going to happen quickly, and certainly not before the next UK election.