[mdjlucan]

When is the best time to sew the seeds in small pots?

[New shoot]

They grow very quickly and can’t be planted out until the risk of frost is over, so I usually leave them until April.

[Subversive_plot]

This might be helpful: https://www.theenglishgarden.co.uk/expert-advice/gardeners-tips/growing-sunflowers/

If you are later growing them in the ground, I would wait until the predicted last frost date (for your area) to sow in pots, then plant out as directed at the website.