I gather that the D pier blocks the view of that area from the control tower, and any use of that runway would therefore need an additional control facility. But apart from that, I've often wondered why that runway couldn't be used at peak times for smaller aircraft departures. Is the surface condition too poor? Is the DAA (sorry, "daa") reluctant to display any ability to increase peak capacity lest it weaken their case for a shiny new full-length runway?
Don't think that the tower is an issue but a new tower is a must with new runway as it blocks visibility. 11 could be reopened with little work. It was included in the CAP charges (2010-2014) as a provision if needed.
They can get a lot more movements from 28 during peak times and they have used up some of the slack between 06.00-06.15 more efficiently this summer to manage capacity.