I couldn't tell you when it was Dave, two occasions in the last six months. The first time he was about a mile west of me, the second right overhead and as I say, certainly well under 1000'. On the first occasion Biggin seemed to be below decision height as several bizjets had flown go arounds and were holding over Kent and one, a PC-24, passed over me unseen in the clag while diverting to Farnborough. I was just curious...
Just found the regs which say AGL which suggests he was certainly too low - the only helicopters I've seen as low as this was are the police (who mostly stay higher) and the air ambulances.
Except with the written permission of the CAA, an aircraft flying over a congested area of a city town or settlement shall not fly below a height of 1,000 feet above the highest fixed obstacle within a horizontal radius of 600 metres of the aircraft.
The French have history over here - about 35 years ago a French MS.500 flew over the common at about 500'! Very easily read off the reg with the naked eye!