UFO Sighting in Hertford, on August 9th 2015 – Looked like a bright light about 15 to 20 foot across

It was a warm evening and I went outside in my rear garden to cool and have a coffee and I live under the flight path for Stanstead airport and sometimes as they are coming into land the planes put their head/landing lights on, I could see this white light coming from the south west heading towards me and at first I thought it was a plane and it got nearer and nearer and I could not hear any engine noise and it was on a curved flight path at about a height of 800 to 1000 yards above me and as it reached me and was heading east a passenger plane coming from the south cut in front of it and just above it and I could get a scale of this light/ craft as about 15 to 20 feet across, it was like looking at a dazzling head light front and rear and side, it was travelling about the speed of a single engine prop plane but this was no plane and it flew off in approx an easterly direction just north of Harlow Essex, I tried reporting it to Stanstead Airport as a near miss but they just gave me a link and to date nothing heard, the passenger plane missed it by about 200 yards which I know is close in aviation terms, I was just shocked to see this thing flying past me and just froze and watched it disappear in some low cloud to the East.This was about 10 pm London time give or take a few minutes on Sunday the 9th August 2015.