The stack does not get cleaned any higher than the bottom of the first floor, as it would cause flooding.
The internal plumbing is the responsibility of the leaseholder, however we have just had maintenance carried out on a top floor flat in 1RQ due to a faulty durgo valve.
Perhaps speak to Rynew and have their maintenance chaps attend. You will have to cover the cost of inspection but Fred will have a better idea than someone else as he is familiar with the pipe work at RQ. Access to the rodding point may be required to clear any blockage discovered in the stack.
Hope that helps.
Marie our property agent will be back at Rynew on Monday 1st October as she is currently on Holiday.