Christmas Potatoes - sorry I know it's too early!

I know it is early days yet, but I am thinking ahead to a Christmas Potatoes project.  My plan is to keep the poly greenhouse up in my back garden. I have two huge plantpots – one fits inside the other with about an inch all round. 

I plan to fit one inside the other and pack the space between with bubble wrap, leaving drainage routes at the bottom (but insulating the bottom as well). Then placing the whole thing inside the poly greenhouse as the weather closes in.

I gather people also put a clear poly “tent” with ventilation holes over the plant for more insulation.

I will be using peat free compost and feeding with Comfrey extract in the watering.  Do people keep the pots a bit on the dry side to prevent rotting and moulds?

Also, I wouldn’t want more than 2 or 3 seed potatoes. Can I buy in such small quantities anywhere?  I know a few fellow allotmenteers have used their own produce as seed potatoes or just bunged one in from a supermarket, but I have reservations about this approach.

Thanks mumofstig - good points. 

The fridge - of course. thanks aelf. I think, if I cannot find a cheap source of Christmas potato seeds I will try a couple from my own harvest; maybe one from supermarket, but prefer my own to be honest.

goodegg - how do you 'let them go a bit green'? does this involve leaving them out in the light after harvesting? then pop in fridge for a quick fake winter?

aelf
Thanx for the tip what a brilliant idea i shall save some of my charollettes too.