Operation of Aircraft Without A Noise Cetificate
Cogwheel
Here is the ref you were looking for:
"Permission is hereby given for that aircraft to engage in air navigation without a noise certificate.
Permission is given under Regulation 9A(2)(a) of the Air Navigation (Aircraft Noise) Regulations, i.e. the extent to which the aircraft exceeds the aircraft noise standards is not significant.
This permission does not extend to operation of the aircraft at a curfewed airport during curfewed hours"
Extract from the letter allowing my Experimental aircraft to operate without a noise certificate. This permission must be obtained before a Special Certificate of Airworthiness is issued for an Experimental aircraft.