Recurring sewage backflow in a leasehold duplex flat in a council building

I’m hoping to get some professional advice from plumbers or drainage engineers about a recurring drainage issue.

Property layout:
• Purpose-built council flat building
• Two duplex flats stacked vertically (each flat has an upstairs and downstairs)
• My flat is the lower duplex
• The flat above has the same layout
• There is also a neighbouring flat to the side on the same stack

Bathroom layout in my flat (and neighbour above):
• Downstairs toilet
• Upstairs toilet
• Upstairs bath

Issue:
I have now had three incidents where sewage has come up through my downstairs toilet when the drain blocks.

The pattern seems to be:
• A blockage in above flat which council plumber comes out to remove
• Drain downstream becomes restricted
• Waste backs up in the stack
• My downstairs toilet becomes the overflow point

Most recently the drainage company jetted the drain and removed fatberg-type deposits and debris (grey mineralised grease and solids). A plumber had also attended the upstairs neighbour just before this and said there were wipes blocking their toilet, although I realise wipes and grease can come from any of the properties in a shared system.

Mitigation so far:
I’ve installed temporary flood protection around the bathroom door (door cover, absorbent socks, etc.) and wet-vac the water while waiting for drainage engineers, but obviously this isn’t a sustainable solution.

The council's drainage contractor already came and did a CCTV and clean out before Christmas. So either it's build up again or they didn't get everything out.

My main questions:

1. In a building like this, would installing a backwater / non-return valve on my branch pipe be a possible way to protect my flat from sewage backflow?
   1. If the council are responsible for the pipes, is there anything I can do internally or do I need to get them to do this?
2. If a valve is installed and the drain blocks again, what normally happens hydraulically in the stack? Would it simply back up to the next fixture upstream (e.g. the upstairs flat) or spill somewhere else?
3. Is there any risk of increased pressure or damage to my pipework if my branch is protected by a valve but neighbours continue using water during a blockage?
4. Are there better engineering solutions in a situation like this that are commonly used in lower flats?

The drainage company has suggested the issue may be recurrent fat / debris accumulation in the shared drain, so I am also considering whether regular preventative jetting or a CCTV survey would be worthwhile.

I just want to prevent future flooding in my flat and understand what solutions are typically recommended in shared drainage systems like this.

Any advice from plumbers or drainage engineers would be hugely appreciated.