There are lots of weedkillers and moss killers you can use but the trouble is that the moss will always return unless you tackle the cause of the problem. This is that the ground is too damp or poorly drained. The only way to fight this is to spike the lawn, brush sand into the holes or even take more radical measures to improve drainage.

Many thanks Bertiefox I will try with the sand.

Increasing the pH with lime should also help, but Moon_grower tells me that this takes some skill.

many thanks Tugbrethil