Too many aircraft trying to arrive and land at the same airfield at the same time. It happens at every airfield sometimes (even the much vaunted AMS) because demand is higher than capacity at that time.
The reason there is always holding at EGLL is due to lack of runway space, lack of airspace to stream in and that it is always operating at close to capacity. One little delay of any sort can cause problems.
Somebody mentioned before in one of the chats that London is developing the software to act on the A/C coming from the Europe in order to "stream "them. I have to say that this is exactly what CFMU has been doing for the last 15 years. I have a feeling that CFMU and London should sit down and try to find compromise on how to go ahead in Air Traffic Flow Management for the future
Sorry if i don't hold out much hope for this. If CFMU were so good at creating a stream for EGLL please tell me how you manage to let 8 inbounds get to the same point at the same time? The only way to create a landing order then is to hold some so that they can be put into stream the correct distance apart. I know flow is and can be a inexact science but surely letting an hours worth of traffic turn up in the first 20 mins of that hour shows theres something wrong?
The new software for the LTMA will only be a good as the people using it. What happens if the kit tells me that ABCXXX should be infront of BCDXXX and its physically impossible to do that?