potato disaster

I have one compost bag with first earlies growing in, and one of main crop.  Yesterday I was thinking to myself how huge they'd both grown- massive plants exploding out of the bag.

This morning my first earlies had completely wilted and fallen over (not blown over I don't think- it was pretty calm here last night).  I propped it up as best i could.  This evening I came home... it had been completely eaten by little slugs.  They'd chewed completely through the main stems just leaving stumps and everything had started to shrivel.  I was maybe a bit hasty, but I decided to cut my losses even though I think it's massively early and managed to unearth 4 little potatoes, then chucked the dead looking plants.  So disappointing! 

Was it something I did or didn't do, or just one of those things?  I don't want my main crop to suffer the same fate!

I have potatoes both in grow bags and in the ground the ones on the allotment have first been hit by frost then devestated by slugs, the grow bags are doing great. To me the advantages of bags is you can start early, bring them indoors if frost threatens but also it is easy to protect from pests, pick your method pellets work or if you dont like them a copper ring round the bag should work fine. The great thing with bags is you can pop out to the patio rumage in the bag and you have fresh spuds in the pot in a couple of minutes, then leave the rest to grow on lovely