Chopjock,
If a non-certified part is fitted, who is to say whether the aircraft is safe to fly or not? Ask any aircraft insurer for their views on the matter. Ask them if they would insure a helicopter if non-certified parts were fitted.
The reason the certification of aircraft parts has been mandated (it's not just the UK who do this, of course) is that in the past things have come apart in the air. How would you feel if an uncertified (and therefore illegally flown and probably uninsured) helicopter fell apart in the air and landed on your house and/or family?