Why are my sweet pea buds all withering? :o(

[plipplops]

Title says it all really. I've got some sweet peas in a pot that I've kept well watered (have had problems with powdery mildew in the past so was determined to keep them moist this year). Now loads of the buds are dying off and I have no idea why. Any ideas?

Something to do with the soil, but can't remember what sorry?
Hopefully somebody else will come along with more info.
Are they getting enough sun, this has stopped the mildew in plants we had at our old house.

Yes it's South facing so plenty of sun, and new compost. It's not happened before, I'm hoping it's a temporary glitch of some kind rather than it all dying :o(

Hmm, didn't mean to do that face...