Hi Louis,
That video of sowing sweet pea's must be one of the worst ive ever seen,
Firstly if you look at the pods thats been cut off the main stem, the peas have already been opened by the plant itself (that twisted pod)
Half the seeds have gone,
If as i pointed out those stems had been removed just a little ealier and put into a paper bag hung up side down and put in a dry dark room you'd have a lot more seeds? hence more chance of plants?
Now the sowing,
Ive not seen sweet peas sown that early, store them like the seed companies do and plan a sowing later on ,
Help the seeds to open by using the overnight soaking i mentioned,
The way the video did the sowing (1 x seed per square) How do you know if thats a good live seed or a dud ? AND 1 X seed per square i think is far to little, i'd have 4 or 5 in that same square, and by using the soaking method you'd know what seeds are alive and whats a dud seed,
The actual depth that 1 seed was sown was not deep enough,
And the perlite was nothing like enough, or spread across the complete area of the square potting tray,
And last but not least, stand the seed tray in water and let the compost get moist from the bottom up wards,
If you water perlite it turns green,