As mentioned above, 26R/08L is used as the spare runway to cover runway maintenance, etc.
I don't think the extension to the South Terminal has made any difference in terms of ability to use it, but because the runway and approach is relatively close to various parking stands, there are a number of requirements before it can be used, so it cannot be put into use at very short notice. e.g. Stands 140- are restricted to smaller types, etc. etc.