I would imagine slots are required to restrict the planned toal number of of arrivals. CAA?
On the day there are always many less than the slot list, due to the vagaries of the weather and a/c availability etc.
And of course, many pilots book a slot for each day and then only use the one that fits, for the reasons above.
I've always found the Sywell ATC very helpful in reallocating slots for cancellations etc.
Contrary to the doom merchants re the arrival procedures, any problems I have had are caused by non adherence to the Notam, and rubbish airmanship,not total numbers arriving, this also applies to the LAA rally.

Correct! But no matter the rules, there will always be idiots who ignore them or have not bothered to read the NOTAM. By choosing to go 'no RT' Sywell make the problem worse. More control and less free for all may sort out those who are undisciplined.

Well the Xwind was over my limit on Friday, however a friend kindly drove and I re-booked to fly in on Saturday a few minutes prior to the late morning cut off with very few aircraft booked at the same time.
Shopped and met folks on Friday and the drive was not bad at all.
Checked Saturday's booking and instantly cancelled as other aircraft booked on every minute leading up to and after my slot and even one at the very same minute.
Decided with only one set of Mk I eyeballs to look out round Pitsford it was best to blink.

But... There's a map, using a Google Earth image! How much clearer could it be?!

GK430. For the reasons given previously the published slot list bears no relation to the actual a/c arrivals. I flew in Sat 1200 ish Pitsford direct to downwind. Amazing visibility and no problems.

We had no problem flying the correct hold having studied high def detailed maps in conjunction with the one on the procedures instructions. The trouble was partly caused by the extremely poor definition of the published map, which many pilots obviously took as an excuse not to fly it accurately.

Thats my point exactly - a small google map image led to some interesting interpretations on Sat - some virtually over the reservoir others on 747 size circuits around it!