We used to apply a liquid 6X, nettle and comfrey fertiliser, as I always felt that as the roots are going to be so far down, a liquid feed would get down there quicker/easier than a pelleted version!
This may be a flawed argument, but the leeks used to romp away every time!
One tip I can offer, is to apply a liquid around the roots and never over the leaves, as the collected stuff in the channels, over a season, can discolour the veg somewhat!