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Doggit
From the look of that blowback, there's obviously been a blockage downline somewhere, and the high pressure cleaner has pushed the crap back up your pipework. The upper flats may not have been affected as the are higher up. This definitely isn't down to your refurb work. How did the flat below you get on, and have they had the gurgling problem as well.
Looks like you've got away lightly, (possibly as you're 1st floor), known the entire room need redoing after we changed it to a shade of brown.....
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Doggit
Under the circumstances, I'd fit a Hep20 to everything in the bathroom, just in case of future problems. I had a property whereby the basement toilet always overflowed because people somewhere along the drain kept throwing toilet wipes down the drain and blocking it. Hopefully, the drain guys will have cleared the blockage, but I suspect you may get future problems before this gets resolved completely.