Leeks can be sown in a seedbed or in modules or in pots.

We shall sow in a single pot, about 20 seeds in all, in late March/early April.  They should begin to show a couple of weeks after sowing. When the roots have filled the pot we shall then place the contents of the plot in a holding bed where they will grow on before planting out in May or June.

This method allows for germination under cover but growing on in the open ground. They could just be sown directly into a holding seedbed to grow on before transplanting.

You will need 6-8 plants for each 1.2m row; plant one or two rows.

The RHS has a very good video on leeks.
https://www.rhs.org.uk/videos/advice/Leeks

I tend to plant out my leeks when they are much smaller, with no need to trim roots or tops, and into shallower holes - 6-8cm deep.  When the ground is dry it is a good idea to lever a small gap with a trowel and water into the gap; using a dibber in a drought is impossible but pre-watering makes it easier.  There is a dibber on the compost heap.  I also ignore the advice on needing lots of white stem.

I plant about 20cm apart in rows 30cm apart but I have the space and want large leeks in late winter.  Where space is limited and cropping is before Christmas 15-20cm each way is ok.

Charles Dowding offers an alternative approach, including planting less deeply in clumps and with no regard for getting white stems.
https://www.youtube.com/watch?v=CrqcUK2rdhw

The mesh covering is to try to avoid damage by leek moth.  I had much damage a few years ago so I now mesh from planting time until September.  There is mesh in a fabric bin in the 251 Shed.