The n/s rear window has started to go down on its own whenever I close it? Only way I can close it is to take it up gradually bit by bit. All others ok. What could cause this, is it something to do with the mechanism??

Mine used to do that. I think its caused by the window control unit thinking there is an obstruction and lowering the window. I cured mine by opening out / lubricating the top rubber channel.

You need to re-calibrate the window regulator. that will fix it. instructions in your hand book.