Hope I can help, Rowland!
'Krypton' leeks (Unwins/Marshalls seeds) sown in a half-tray during last week in January, potted on around 30+ (that's most of the few seeds in the packet which germinated properly), into two deep 6" pots during the first week in April, then, at 'thin pencil' size, dibbled into their final bed during the last week in May.
I guess there's a need to start them off much earlier than March. I grew Musselburgh last year, on roughly the same timing, but they didn't get to pencil size - never have done with me, but they went in as usual, and were fine!
I like the idea of earthing up leeks, especially if they extend the blanched section, that's a sound method, so your FIL knew his stuff! I dibble holes to around 8" and about 8" each way for the thirty plants in a block, and it seems to work! Some are still much smaller at the moment, and probably won't get much bigger now.
When we had 'The Patch', I used to grow around 200 plus, but finished up giving loads away at Easter...
Frankly, I just love growing the blighters, they're so forgiving and down here at least - don't seem to be worried by the nasties!