Tony Eva wrote:
I need to get hold of some silicone sealant for using on part
of the bathroom floor, but the snag is that aesthetically it
needs to be grey. The usual sources (the sheds, Screwfix,
Toolstation) can do white, clear, and brown, but not grey.
Does anyone know where I can find some?

You can colour sealant.
All you need to do is to find a compatible colour, and mix it in.
Take some white, and add some carbon-black, and it goes grey.