Question, where are you north south as different sowing times will apply.
What are you sowing, seeds need differing heats and propagators usually have a standard cooler setting, a thermostat gives more scope.
What exactly did the GC advise when selling you sand and grit and did you get the sowing compost too.
My seed mix is one third compost one third grit and one third washed sand and yes it needs be damp although not wet.
Seeds differ in germination times, some will be up in hours others take weeks, depends on what you are sowing.
So more questions than answers, it is always best when asking questions to state what you are growing, in my part of the country I am sitting with my feet up as it is far too early to sow most seed and heating a Greenhouse even a lagged one costs big money, it is best to heat just the bit you are using, easy to do with shelving and curtains of bubble wrap.
Come back with some answers and we will be willing to help.