We had all the main runs pressure washed from the garage. From past experience blockages occur on the horizontal run and not the vertical. Thus the most common point of blockage is the horizontal run from the communal stack to the Thames water drain via the garage. It was these runs that were recently jet washed. I suppose there may be a remote possibility that there is a blockage at just the top of the soil stack but this would mean it would affect all water drainage for both top floor flats.
I am not familiar with the plumbing but it seems logical to me there would be a stack for kitchen, bath, shower and basin waste and a separate stack for the toilet. Most often it is kitchen waste that is the culprit as people assume you can flush anything down it.
I looked through your previous posts and I see that you had a similar issue in 2015 that was resolved by a second visit by a plumber.
If you don't have any luck with the domestic plumber than I would try a company called DCPM who have looked after soil stack issue at RQ in the past. Rynew should have the contact details. They can be difficult to get in touch with so Rynew may advise a different contractor.
Hope that helps.