<<how do you prevent aircraft making their way to final approach for the 'wrong' (no longer in use) runway>>
Hi Nick. All aircraft flying into Heathrow - and the majority of other UK airports - are under radar control from take-off to touch-down so they do not make their own way to the runway. They are given instructions on altitude, speed and headings to fly to produce a landing sequence which results in a very high landing rate, even in poor weather conditions.
When a runway change is decided at Heathrow a specific time is promulgated for the change and various agencies advised. All being well that time will be the last landing in one direction followed very soon by the first landing from the other direction. The radar controllers who are directing the inbound traffic are told well in advance - maybe 30 minutes or so - of the time of the change so they can calculate which aircraft will be the last lander before the switch over. Meanwhile, they have already directed traffic off the stacks for the opposite landing direction.
The radar controllers are in constant contact with the Tower controllers and will be advised of, say, the last 4-5 departures. As the departures follow specific routes the approach radar controllers can liaise with the departure radar controllers to ensure adequate separation between the "first landers" and the "last departures". It may sound complex but it usually works quite smoothly.
There is usually a short delay during the change-round but rarely is it significant.