Black Triangle Sighting in Chelmsford, on April 11th 2015 – Black triangle slowly spinning c/w with maroon 3 dim lights 1 on each corner

I was watching the ISS fly-by. It was visible 273 degrees (W) and due to fly over above starting at 21:34 ending about 9 minutes later. I was using my iPhone App, ISS Spotter. I had not used the app for at least 3 months but earlier that day as it appeared to be going to be a clear sky I thought I would set its alarm to remind me as there was a viewing opportunity that night. At the time (late afternoon) I was considering using my Nikon D700 camera with 300mm lens to photograph it but thought I wouldn���t bother as it would just probably end up being a white dot on the screen. I was working on my PC when the alarm went off possibly 5 minutes before the ISS was due to fly over. I went down stairs and asked (half jokingly as I knew they would say no) my wife and daughter if they wanted to come out and see the ISS fly over. I was right they were watching a film. I was in my garden and using the app located where I should stand to get the best view of it as I���m surrounded by houses. To the predicted second I saw the ISS in the west at 273 degrees about 40 degree elevation. Just as it peaked above my head I moved around to my patio (10 feet) to watch it go over my house directly above. However, at 21:37 as I was looking directly up my gaze was diverted to 3 dim maroon coloured lights moving directly above heading south direction at a low altitude. Possibly 1500-2000 feet north to south! These 3 round dim glowing red lights were silently moving in a perfect equilateral triangle formation with one of the points trailing behind. Each light was the apparent size of a traffic light viewed at 50 feet away. I saw the object for about 6-7 seconds as it flew overhead. Whilst I could not see the object, it blacked out the stars thus giving the illusion it was a solid triangular shape. I was also uncertain of its altitude until I notice the perspectives were changing. Suddenly from an equilateral shape of the triangle narrowed and the lights were no longer round but looked saucer shape (like an upturned saucer) bulging from underneath. Once they passed above and were at an elevation of about 55 degrees heading away from me in a southerly direction I noticed the trailing red light moved towards the right leading light as though it was going to barrel roll around it, again this was the illusion it gave but after consideration and trying to make sense of what I saw (as I have no explainable reference to compare it too)I think the object was slowly spinning. At that point I ran indoors to get witnesses (my wife and daughter) but when they both came out 10-15 seconds later it was gone. The lights were not really bright but in size they we about a quarter the size of the moon. The object was easily dismissed by its unique form and silence to be an aircraft or aircrafts and yes, it was silent! The size of the triangle if at 1500 feet away would have fitted inside the constellation, the Great Bear stars that form its quadrant (would make the craft 500-600 feet across, meaning the glowing 3 lights would have been about 30 feet in diameter). If that makes sense. The speed if at 1500 feet I would estimate to be about 300 MPH but not sure except it was fast. I regularly look at the skies generally I can often see satellites, shooting stars, commercial airlines, light aircraft and helicopters flying at night as we have the London flights go past. I���ve even seen Chinese lanterns. The object I saw was heading north to south but there was a SW breeze that evening and therefore was not being blown by the wind. The object I saw was totally unique. I would love to see it again! I hope there are other witnesses who saw it to collaborate this statement. Research on the Internet lead me to believe this was the top secret TR3-B flying triangle operated by USA.