growing seeds in a polytunnel - help !

It depends on what seeeds and the temperature in you tunnel, at night in particular

Tomatoes no, broad beans, yes

But tbh it is really still very early for sowing much, so I would give it around another month

Later sown seeds tend to grow into stronger plants as they don't face weaakening conditions as they fight to grow. They catch up and crop at around the same time for most things.

Indoors in the warm though, you could sow chillis, as they need a long growing season to produce a decent crop, so it is worth the effort of providing heat and that all important light too!!