What's the best time to plant my seed potatoes?

I have one of these http://www.patio-plants.co.uk/images/patio_potato.jpg and I have seed potatoes, which are currently sitting on my window sill in an egg box 'chitting'

Would the end of March be too soon? They are either earlies or second earlies, and I can wrap the planter in fleece if necessary- I can also earth them up a lot which will be some protection from frost as well... which reminds me, another question... when you earth them up, do you cover the stems/ stalks/ chits/ whatever they are called, or do you leave the very tips unearthed?

I know nothing... lol.