Sweet peas gone s?

Good evening

They'd grown to six inches, dead chuffed I was, coz the garden's not been getting much sun, and all of a sudden they're dying off. The stems seem to have gone all skinny/mouldy at the point they are coming out of the earth. They're in large terracotta pots of compost, not beds. Any ideas on where I've gone wrong?

 

It is because they are in pots I think, they need the moist cooler soil of a bed to flourish well.

 

Hi Jackcat ... well you did better than me as mine bloom in the Spring here and I had NOTHING show up and I only do the black/purple ones ... so disappointing this year but it was our wettest winter in over 30 years so I don't know what went wrong ... drowned?

 

I've a feeling I may be over-watering 'em