MM wrote:

The silicone sealant (from a gun) behind the kitchen sink (sealing the
worksurface to the wall tiles) has several spots of mould where I have
been a bit lax with the cleaning. Anything I've tried will not shift
the mould stains. The mould seems to have ingressed into the sealant.
Is there any modern product I could try? Or is this a case of redoing
the sealant strip in due course? I looked in Tesco, but they had no
mould remover of any kind.

The only thing that will work is replacing the sealant. The mould is
often partly within the body of the sealant so no amount of chemical
warfare will get rid of it. The problem seems to be worse with silicone
sealants that produce acetic acid as they cure. Several species of
moulds can use this acetate as a food source, and the mould can
penetrate the sealant where it is cosy, well fed and happy.

Neutral cure silicone with a fungicide doesn't suffer in the same way.

Most of the commercial mould cleaning agents are based on sodium
hydroxide and bleach (sodium hypochlorite) with a little detergent "in
variable proportions". In general if you don't know much about chemistry
I can't recommend experimenting since the mixture causes terrible eye
damage if you manage to spray it into your eyes.