Potatoes - when to plant

Ive laid all my tates out in the greenhouse, and if the temperature drops near 0 ill light the heater.

Im working on the theory they chit and should be ready to bung in the ground  mid march.

I thought Thompson and Morgan were blooming expensive. I bought 5Kg of each (1st early, 2nd early, main, late) and it cost me £12 at the local lotty shed, T&M wanted £39 for the same deal.