Just wanted to mention that reputable stock would not have prevented this excitement. It was running out of the soil you wanted to use.Ok guys...your questions have allowed me to solve this mystery. As I was typing about how I propagated the seed, I remembered one important detail....I was lacking around 2-3 cms of topsoil on all of the pots when I filled them and used the soil from an old pot....forgetting that 18 months ago it had a celosia in it which I ripped out after it died.
Soooo....obviously none of the cannabis seeds came up and I have been covertly hiding my celosia growing from my neighbours. I am horribly embarrassed lol.
One good thing has come out of this. I will indeed get reputable stock next time. Thanks so much for your responses and your warm welcome!
It is possible to save a few seeds. The 2-3 cms of soil is about an inch. What I see as something to try is to remove just short of the amount of soil off the top of the pots after carefully pulling the weeds. In other words, just take off a little bit less than the amount of additional soil. (Any plant growing in an undesired spot can be considered a weed.) This will reduce the amount of soil above the Marijuana seed and it might start to germinate.
Then if whatever might come up has the sawtooth or serrated edge on the first true leaves then you are on the way. Or, you could end up with more Celosia and would have to start giving them away to the neighbors.