The Wycombe Air Park period of silence was not NOTAM'd but promulgated by ATC via letters to all airfield based operators and to private owners, visitors etc by letters on the airfield information board in the airfield reception area (alongside NOTAMs, met, airfield info etc).
A note was also attached to the top of the booking in/out form. In addidtion, ATC were warning aircraft that were already up and running.
Unfortunately, ATC can't stop people who don't/can't read and start their engines after the silence period has begun. The aircraft in question would probably have been out of sight down that cul de sac...