Slugs feed more at night than during the day so it could well be them. You could try a copper band around the pot to deter them.

Here in the USA copper tape is sold in the garden supply stores. It is sticky on one side so it makes it easier to apply around the pots. It is supposed to deliver a little electrical shock to the snail making it unpleasant to slide across its surface. MG has made a great recommendation for potteds otherwise it's the good old beer traps for the garden.

Copper tape has worked really well for me in pots. Copper bands are also good while the plants are young but once they get bigger or surrounding plants overhang, the slugs will find a way. They have copper tape in Poundland.

lift the pots up and look for the small slugs under the pots. Knock the plant out and again look for slugs. you can use slug pellets sparingly if you want too.

Lupins and dahlias are just not worth the effort here. I resent paying good money to feed molluscs...

I agree Stera and when they do flower they are attacked by monster greenfly, you can't win.

I've never had aproblem with slugs and dahlias but they are all in pots, protected by copper tape which seems to be effective for years. Lupins are another story.