when to plant my seed potatoes

I won't consider it until late this month here. I would suggest that you might be looking at waiting for another month where you are.

John may be the best to advise on this - he's just across the water!

My neighbour is Irish, he has a saying... "In with Paddy, out with Billy" to translate... in on St. Particks day and out on William of Orange day.
I used to go with this until one year I decided to try and stagger my earlies, the first row was planted on St. Patricks, the second row was planted on the second weekend of April, guess which row I started to dig first? Yep the April one!
All my spuds now go in on the second weekend of April, unless we have a superb spring I will be digging them within a week or so of those who planted in March!

Illogical, I know, as Easter is a moveable feast but convention around her is to plant on Good Friday

I think I say this every year, but even my dear old grandfather (he was a gardener in Gentleman's Service) would say this maxim was nonsense. However, this year it works out at around the right time to consider it.