The pool has been open for less than two weeks, and I noticed that the sides had become dull and discoloured over winter. Just below the water line it had become slightly beige-ish and dirty looking.
I tried scrubbing, but nothing happened. Then I remembered our little trick from last year; Vitamin C.
I emptied a Kilo pack of Ascorbic Acid powder into the flow of the water jets, and within an hour the pale blue liner was as clean as the day it was installed.
You might see from the photo that everything is now sparkling clear (on reflection I should have taken a 'before' photo).
If you, or your friends, have the same problem with grubby looking pools, remember Vitamin C.
On top of which, we probably won't catch a cold for the next ten years!!!