My god.........words fail me, for once.
That looks like it has been a very very bad DIY job..............20 years ago.
In my opinion..........(and I like slagging BnQ) you can't really blame them. Once up, ya stuffed...........several boxes should have been opened and used.
As mentioned..............stop now and rip out and start again.........with a Pro
tiler, not some half baked "I can do that" merchant, because obviously he can't