Potatoes-vandal or disease issue?

Hello

A few weeks ago I planted a brand new bed of seed potatoes.  The ground has been fallow for some time but has been well weeded and a good amount of moisture.  They were doing quite well and I was earthing them up as they needed it.  One day I noticed some of the stems had snapped but I put this down to our idiot cats having a bundle in the new bed, however I did note a few blackened areas on the stems but again I assumed this may have been damage courtesy of bundling mogs.

Last saturday our house was burgled with entrance gained from our allotment.  A midst all the commotion I didnt look at the potatoes till last night; every single stem has snapped in two.  Again I notice blackening on the stems. 

My question is this vandalism (human or animal) or something like black leg?  The plants didn't appear stunted at all.  The next question would be do I dig them all up or leave to see if they recover? And if I do dig them up could I get away with planting another potato crop straight after or plant another crop?

Cheers