Cillit Bang black mould remover.

[VicS]

It's basically bleach in a fancy bottle.

http://www.crystalcleaningsupplies.co.uk/images/20190718104834Cillit-Bang-MM.pdf

's right
Reckitt Benckiser list the "ingredients" as

Ingredients

• Aqua
• Sodium Hypochlorite
• Sodium Hydroxide
• Alkyl Dimethyl Amine
• Sodium 2-ethyl Hexyl Sulfate
• Parfum

 

[PCUK]

I've found that Barretine brush cleaner works almost as well without the fumes.