Just another bit on VORTEX in mixed mode. It's fabulous if all the traffic from the north lands on 27R and that from the south on 27L; then we could really pack 'em in. Unfortunately, as has been proved many times, traffic into Heathrow comes in chunks (!) - sometimes offering from the north, later from the south. That means that we would often be faced with a "crossover" situation - north traffic landing 27L and vice versa. That's where VORTEX problems arise in IMC, which is usually the case in the UK. In IMC the traffic crossing behind has to be at least min VORTEX horizontal separation from the one ahead unless it's 1000 ft higher. If you go for vertical and #1 doesn't descend fast enough (the guy waiting for the glideslope when he's been told "descend altitude 3000 ft") somebody driving a 744 after a 13 hour flights gets popped on at 10nm at 5000 ft. In VMC we can use visual separation and go down to 500 ft vertical for VORTEX, but that doesn't happen every day.
Under certain holding conditions we can land on the departure runway to reduce delay but many is the time the Tower has complained that we have not achieved any benefit simply because traffic has been slow to descend so the one behind for the other runway could not be descended quickly enough to meet the glideslope.
If Mixed Mode is ever trialled at Heathrow both ATCOs and pilots will require a degree of education.