BT and all interested parties: I have now found three relevant references.
1. My old CFS notes (circa1980) say that the first false GP is "not below 10deg".
2. "Ground Studies for Pilots", Taylor & Parmar, 1990 quotes "above 6deg".
3. "Air Pilot's Manual", Thom, 1990 says "above 12.5deg".
So, I don't think I've proved anything, but it would be wise to assume the lowest figure until we've proved otherwise. Is there a "Calibrator" out there?