D to A,
I imagine the first major problem with your plan to extend runway 23/5 is that when 27L is being used as the departing runway, all aircraft from terminals 1,2 and 3 would have to cross the now active runway 23, to get to the threshold of 27L.
Also, runway 23 is used as a holding area for 27R departures, usually for T4 aircraft and some T 1,2 and 3 heavies. On regular occasions, when the queue to depart is long, the 'traditional' holding area at block 134 goes out on to the outer taxiway, so by the time you add all the T4 aircraft to this queue, you will have very congested taxiways around LHRs central area!
Also, lets not forget having to get your timing perfect so that an aircraft isn't landing on 23 just as one comes overhead to land on 27R, or an aircraft on 23 has brake failure and collides with an aircraft rolling down 27L.
[This message has been edited by splonguk (edited 29 June 2001).]