I am a fountain pen enthusiast and there is
one shade of intense blue ink which is famously high staining. Well, Guess what got on my white Apple Watch band? I was cleaning a pen and it wiped itself off on my band and I didn't find it for hours. Great, now my band is stained forever. Goodbye $50 USD.
BUT I tried one fix, in the US we have something called Mr. Clean White Eraser sponges. They get out just about anything. They are great for things like wine stains on porous white Formica counters and the like.
So I took off the band from the watch and went to work on it with the White Eraser. After about 15 minutes of scrubbing the stain was gone. The band is stain free but the finish is not quite as matte as it used to be. I suspect that the eraser has a bit of abrasive in it. However, it is almost impossible to see that unless you know what to look for and you look real hard.
I am sure that they have Mr. Clean White Erasers in the UK. They are like a fine grained man made sponge and other companies have copied the idea and put them out in our grocery stores.
Here is a great run-down of how they work.