Sweet pea seeds

Re-sow them during the autumn, then put them somewhere cold but frost free. Be nice plants by March next year. Don’t forget to pinch them out when they’ve reached 3-4 inches Personally I would try to resource the most fragrant ones (usually heritage) as well.

Thank you for that.

Do I resew them in pots indoors? Is it OK to plant them outside in March?

Collect the pods and leave them somewhere to dry, or collect them from the plant when they are dry before the seeds burst out onto the garden. I've grown them this way and they were successful, considering they were from a cheap source they did well and were highly scented.

Re-sow in long pots if you can for their long roots but not indoors. They need it cold so if you have a greenhouse fine or put them in the lee of the house and protect them in the coldest weather with glass or better still if you have a cold frame.

Any seeds you gather should have all the bits of plant material removed then pop them in an envelope and label.
Depending where in the country you are you could sow the sweet pea seeds in the autumn or next spring in deep pots protecting them from freezing temperatures. (You could sow some this year keeping back half to sow next year.) Nip the tops to encourage side shoots.
They may have been cross pollinated and if so will be different from the parent plant.
But some seeds of deeply scented varieties to mix in with them.

That should read Buy some seeds

I have two deep troughs on legs with tall plastic (sort of see-through) lids. If I started them off in there in autumn, should they be in the garage for instance, or outside by the house? Would I need to water them all winter?

Sorry - you are talking to a novice here!

They will need light, so putting them in the garage won’t do. Can you make a cold frame out of a large plastic crate and some Perspex for a lid? Or buy a very small, cheap plastic greenhouse from Wilkos.
I sow mine in toilet roll centres and stand them in a plastic box. They will need to be kept just damp, no more.

Sorry I missed the bit about already having some troughs. They should be ok in a sheltered spot. If heavy frost and snow are expected you could wrap them in bubble wrap for a bit of insulation.

I collect mine when the plant has finished dying keep them in an paper envelope( not a plastic bag) and plant in the spring four or five to a pot if they re going in pots
It’s worked for me every year and I get a great show

Garage has a big window in it, so maybe it would suit. Cold but not frosty.

What’s the advantage to planting in autumn to planting in spring then ?