From (very fallible) memory I think you need to phone the airfield to book the slot. I have this feeling that if you're arriving VFR then there's a bit more leeway about the slot time, but its several years since I last went to the 24hr race so things have probably changed a bit. If you're arrving on the Friday things might be a bit easier as well - there tends to be a lot of traffic on the Saturday and Sunday.
I suspect the the NOTAMs aren't 'attached' to the airfield, but are issued as a general one covering the race, as they deal with temporary CAS and IFR procedures as well. If you can't find anything on the SIA website you can always try gving AIS at Heathrow a call they sould have copies, or be able to point you in the right direction.
Brooklands