I think I'm going to have to do a bad thing.
I harvested my flax a couple of weeks ago. It looked great! But the weather's turned cold before I've retted it, and you need warmth to ret flax... Essentially, you need to give the flax moisture, warmth and time (about 5 days?) and it starts to rot. That breaks down the outer, bark-y bit of the flax and allows you to get at the fibres inside.
I don't know how long you can keep harvested flax before retting it, so... I think I'm going to have to ret this indoors. Rotting vegetation. It will be steeeeeeeenky.
Sorry Mr P.