Doughnut shape basketball size slow flying

We saw one of these over Bournemouth flying from west to east towards the direction of Hurn airport, following the course of the river Stour.  First sighted them over near Longham Bridge.  Tried to take a photo but couldn't zoom in enough.  It looked and flew exactly like this, like a doughnut shape tumbling end over end.  No fans, no propellor, no sound or lights or exhaust.  Thought it was a balloon to start with but there was no string and was flatter than what a balloon would be.  It wasn't flying erratically, it moved as if it was being powered.  There was no wind either.  Never seen anything like it before, it is interesting to see others have seen such phenomena in other parts of the UK and the world!  Not saying its aliens or anything but definitely "unidentified"!