Posted: 17/09/2015 at 08:56
Perhaps it would be best to answer your query step at a time.
First of all, living in Staffordshire I'm roughly in the same place re seed sowing times. I would say that rather than setting your mind on sowing those seeds in early October, wait to see what the prevailing weather is like.....sometimes mid-November is more favourable. Sowing too early will result in the seedlings being too developed going into winter.
Apart from sowing in deep pots such as grow-tubs (I prefer the (plastic bag) type sold by Eagle) I would recommend using seed compost rather than multipurpose.
I like your trellis project, but as sweet peas are both deep rooted & hungry feeders the trough you mention will need to be as deep as possible.
Those dwarf SPs would be better sown in the spring, then sow as with others.
Regarding those packets in your pic, the only ones I have experience with are Unwins, who are a reputable company who have always sold good quality SP seed.
Could I just add (I've posted this before) if anyone has any queries about growing sweet peas, could you please use this thread rather than send them in a PM to me...it makes sense, as everyone then shares the information.