Hi Jack, thought your community might like to be aware of the following regarding composts.
Bought a bag of Melcourt SylvaGrow multicompost back in February and used it for seed germination. Within a few days small toadstools appeared over the surface of the compost. Looking unsightly I also found picking them off disturbed some of the seeds. I contacted Melcourt who were very helpful. The toadstools are a saprophytic fungus living off the wood in the media and should not affect the plants. They recommend that their composts be used in the season of manufacture provided by an 8 digit date code on the side of the bag. Damp storage conditions can also influence toadstool growth.