RE slot delays.
I imagine it may depend on which airport you go to. I work at an airport which does not use traditional paper flight progress strips, but uses what we call EFPS (electronic strips). When a flight plan receives a slot, the strip flashes on the screen and when you click it, you get a pop up saying the new CTOT and a 'reason' for it. Unfortunately, these 'reasons' are very vague. Usually 'ATC Staff/equipment', 'Weather En-route', or 'destination airfield restrictions'.
On top of this, this reason is also preceded by a code, which if you look up, you can, eventually find out a more detailed reason. Usually down to the particular sector. Alternatively, and the method i tend to use, is ring London Flow Control directly who can usually give a good reason, particularly if its a UK restriction.
Anyway, the reason i say it depends on airfield is because i imagine (having never worked at airport without EFPS) without EFPS it is a lot harder to know the reason at all (someone can vouch for this im sure), you simply get the slot.
So, you can get the code, even at a non EFPS airfield, but the reason you will get from some and not others is simply that it is, at the moment, very time consuming, and a real hassle when busy to find out anything concrete...sometimes with code, it doesnt help!
Feel free to ask the controller, if they are too busy..you might not get answer...no harm in asking. Hopefully with EFPS there will, in the future be more detailed reasons we get.