[beesrus]

It depends where you live.. Down here in the far South west, the mild areas by the sea have long been farmed for very early potatoes. As in Ireland, farmers have always been more worried about our wetter climate and blight at the business end, than frost at the beginning of the cycle. 
In my greenhouse with enough care, the soil is warm enough mid February to plant earlies for a mid May harvest. Outside, middle of March is when I plant earlies if it's not been a particularly cold Winter. Earthing up then gives the potatoes a good 6-8  weeks of frost free worries, by which time there won't be any frost here.

Planting in May is a little late for almost everywhere in the UK. One can be too worried about frost. We're not talking tomatoes/cucumbers/sweet corn.

[Godhelm]

My Rocket went in the ground on 16th March, so it will be 10 weeks this coming Sunday. There were lots of flower buds last week, so I think I might have a go at getting some potatoes this weekend. I planted 20 seed potatoes, so I can harvest them gradually over the next few weeks. Charlottes went in a couple of weeks or so later, so will be looking for them to be ready early July. So I make that 6 weekends to harvest the Rockets: 3-and-a-bit plants per week. Not sure whether the Charlottes will stretch until the Cara and Sarpo Mira are ready in mid-Sep.

[Handy Andi]

Thanks for everyone's replies  I had a little furtle today, around one of the Foremost plants, and could only find teeny weeny spuds, not worth the bother of lifting, so will be leaving them another 2 weeks or so (may have flowers by then too) So will be sowing a couple of marrow seeds today, I'm sure that most of the spuds will be lifted within a few weeks to make room for me marrows 

[Nikkithefoot]

Last year my spuds were finally all in by 27th May. They were very late and as I work full time and with the silly weather last year I wasn't able to get to the plot when the weather was good. At the weekends and when I had the time the weather was carp. Having said that I had one of the best crops ever, late yes but fantastic, even managed to avoid the dreaded blight which thankfully wasn't as rife as the year before, despite more Smith periods.

It is a dilemma, plant too soon and you risk frost, plant late and you risk blight.