this is my rule of thumb
when you are just about all set and ready to plant them and the weather is fine and the world is just peachy
leave it another 2/3 weeks as there is bound ot be a frost on it's way!!!
i was always told to plant them after paddy's day (Mar 17th) but thats far too early for Scotland. I'd leave it until the end of april - and then plant them at 1 week intervals over 4 weeks
this year, i plan to start some very late - July time - but they'll be kepy in containers in the greenhouse - hoping for fresh charlotte spuds at christmas time!
Also, to save my back this year, i'm going to try growing my spuds in a stack of tyres i picked up from the local exhaust and tyre fitter.
simply lay 1 tyre on the ground, fill with compost, stick in seed spuds - when teh shoots come out, add a tyre, fill with compost and repeat for as long as you can manage. At harvest time, instead of digging, just take off a tyre at a time and shake the muck out
fingers crossed!!!!