I sowed my maincrop potatoes in one of the raised beds on 30th March, varieties were Mozart, Maris Piper and King Edward. All the seeds were chitted and planted using exactly the same method, however, the Maris Piper & Mozart have totally failed to grow, I mean not even broken the surface! The King Edward were my own saved seeds from last year and they are absolutely thriving, unfortunately, after digging a few of the others out, the chits are the same size as when they were planted. The MP & M were bought from the supermarket, but saying that, I bought MP from the same supermarket last year and they grew ok?
Some supermarket potatoes are treated with growth inhibitor, some not. Very hit & miss & won't save you money in the long run. Buying good quality seed potatoes is more reliable, safer regards viruses & keeps the seed merchants in business for next year.
Regards when to plant earlies and main crops- they don't
go in early & late, they're faster or slower at growing &
are ready early or late if planted at the same time!
Same goes for other early varieties such as peas. You can sow "early" peas late in the season & get a quick crop before winter because "early" just tells you how short a time it will be from sowing to harvest.
My Grandad always planted his spuds on Good Friday, whenever it fell.
I plant mine after the last lamb is born (also very variable) just because I have time then.
The weather plays such a huge part that they always seem ready about the same time whatever I do!