Don't know if it's available in the UK, but there is a product called Diamond Eye that is supposed to help. Hydrogen peroxide is also helpful. Both have to be used daily to see a difference. The staining won't completely disappear but will be less noticable.
There is also a product here called Angel Eyes that is put in their food but I do not recommend it as it contains Tylosan, which is an antibiotic. I would certainly not like to see a dog on antibiotic long term for what is basically a cosmetic problem!
