I found this site looking for an answer to the same problem, I've cracked it!
Its limescale!I found a product from the HG range of cleaning products called HG Blue professional lime scale remover and it worked with a little elbow grease.
To prevent it returning I use a shower shine with a limescale preventitive.
I tried everything previously except painting it over.
I have use a lot og HG products and found them all to be good, why didn't I look there first!