Thought I would share resolving this experience.
I recently discovered that my toilet was constantly leaking water into the bowl and as I am on a water meter I needed to fix it quickly. I was able to stop the water from filling the cistern by turning a screw on the water pipe that was filling it, only turning it on when I needed to flush the toilet. I got my friendly plumber in who decided I needed a new dual flush valve and I said I would get on for him to fit. However, as he had left the lid of the cistern off I peered in and saw something bluish clinging to the base of the flush valve (my plumber had forgotten his glasses so presumably failed to see it). It turned out to be the wrapper of the blue toilet block (Wilko - I shall alert them) that was supposed dissolve.... I removed this and bingo the problem went away!