In theory, we only get onto 23 when the mean crosswind on the 27s exceeds 25 kts and so whenever there is likely to be any strong southerly wind the Tower supervisor and the traffic manager in Terminal Control will be assessing how likely it is that we will get to those limits. Reason being that just off the end of 23 are the 'Whiskey' stands, usually occupied by BA longhaul stuff. It takes 3 hours notice to clear these and so we have to look ahead. . .. .Once the stands are clear and 23 declared available then we wait for the wind to arrive. As we approach the limits, 23 is promulgated as available on the ATIS and when we get to them we choose a suitable aircraft to make the first 23 lander, inform everybody of the choice and then begin vectoring into the new pattern. . .. .Unless the wind is VERY strong, some traffic (usually the big Heavies) will still prefer 27L and so we have to interweave the two streams. The full Northolt approach is not available at the same time traffic is approaching 23, so they get paired with the 27L landers when there will be a gap on the 23 approach.. .. .Hope this helps.. .. .Point 4