potatoes

I've never grown potatoes before, I have a selection of first earlies, 2nd earlies & maincrop seed spuds.  I'm hoping to get the earlies in next week - new plot so been doing a lot of preparation.  So when do I put the 2nd earlies  & maincrops in, at the same time or do I leave a gap & if so of how long?  My plot is in west Wales near the sea so conditions are fairly mild.