We have an Ideal Concept downstairs which is very good, and was installed by me 3 years ago. At work however we have the same, and they are just awful. Will block if you look at them.
So I'd definitely look at the plumbing/outlet connections.
We also have a Roca Dama-n upstairs, which uses very little water (lot less than the Ideal) and is again very good. It's got a clamshell cistern which is very well designed, completely different to a normal cistern.
Looking at your photo, it appears the black pipe comes up at an angle out the wall. This may be kicking up the pan connector, causing it to back flow into the pan. Perhaps try shortening it, fit a new long pan connector, and see if that makes a difference?
So I'd definitely look at the plumbing/outlet connections.
We also have a Roca Dama-n upstairs, which uses very little water (lot less than the Ideal) and is again very good. It's got a clamshell cistern which is very well designed, completely different to a normal cistern.
Looking at your photo, it appears the black pipe comes up at an angle out the wall. This may be kicking up the pan connector, causing it to back flow into the pan. Perhaps try shortening it, fit a new long pan connector, and see if that makes a difference?