Keeled slugs. You can buy nematodes which you water on the ground and they will attack and kill the slugs. Be aware that unless you are growing lots of potatoes, then it may not be worthwhile. If you rough dig your soil in the winter letting the frosts break it down then you can purge the soil of some slugs. You can try not to grow potatoes on the same ground for a while or perhaps go for the harder varieties of potatoes which the slugs are less likely to attack such as Desiree.

Next year you could trey putting pellets in the trench when you sow them. I usually do this but didn't this year and the difference was very noticeable - lost half the crop. Hate the idea of nemotodes as the slugs take a lot longer to die than with pellets ( 3 days!!)but it might have to come to that.

Thank you all for the advice, will try these out.