I planted my sarpo mira potatoes on 31st March. 6 growing bags had 3 plants each and 3 ten litre pots had 1 each. my growing medium was made up from leaf mould and last year's spent potato bags and a bit of sieved gravel compost. It was all enriched with BFB at a rate of about 1 ounce per plant.
They are all sitting in the greenhouse now. The early sown pentland javelin are still in there too, just showing about an inch above the soil.