toilet
The flapper came off on our siphon flush, so I bought a new one. I fitted it but it was leaking badly.
When I looked properly, I noticed the inner pipe delivering the water to the toilet from the cistern was not flush with siphon flush. This means the siphon permanently fills up with water as below;
Is that normal? Should the connection not be on the outside of the cistern?
If the problem is mineral clogging of the under-rim channels on the toilet, and this does produce the symptoms you describe, then neither CLR no vinegar may be enough to clear up the blockage. The traditional treatment is a cup or two of muriatic acid (HCl 36%, from the hardware store) down the overflow hole in the tank. That goes directly to the rim vents. Pour the stuff in, wait an hour, and flush. Gloves and eye protection are recommmended, and a funnel so as to avoid spillage and corroding the metal parts in your tank. This looks like decent writeup of the method: How to Fix a Slow Toilet
I've run into this same problem several times, and tried the same things you're trying - adjust the ball, bend the arm a bit to put more pressure on the valve, etc.
Ultimately though I've always ended up replacing the fill valve. The pressure from the float ball and arm isn't enough to close the valve completely, a seal is worn out, or something else is similarly out of whack.
ETA: In my experience, the fill valves that don't have a ball but instead have a sort of integrated float seem to be less prone to this problem (like the one pictured below)
Best Answer
I missed it as it was hidden in a decorative metal feature on the toilet.
There is a gasket and a kind of plastic locking plate specifically to tackle this issue.