We've sucessfully grown a large number of lupins from seed and I see
the instructions say not to plant out until Sept. Internet searches
also confirm this.

Frustratingly nobody says why! Can anyone enligten me?

Lupins are very vulnerable to slug/snail attack, and you can lose an entire
sowing overnight (happened to me :-().

Could it be that the idea is to keep them in a slug-free environment till
they are quite big and fully hardened off so they stand a better chance of
surviving?
I keep mine in big pots overwinter and plant out the next year, seems
to work for me :-)
kate