I have been growing my leeks outside for a while and they are now around 30cm tall.
I have read that I should be planting them deeper to blanch up?
Should I pull them up and re-plant deeper or leave them alone?
The conventional method is to make a hole with a dibber and plant them in that (minus half their root length) so they start off well below ground level. I take that a stage further by rearranging the soil into ridges and furrows (like for spuds) and dibbing the holes at the bottom of the furrows. When the plants are big enough you can then move soil from the ridges into the furrows, and later still, heap it up around the leeks (again, just like spuds).
Given that you've planted them on the level, I should just pull the soil up into ridges along the rows as much as poss. That should do the trick. Try and avoid getting soil in between the leaves as you do this as it tends to wear down your teeth when you eat them.