What creature/s ate my delphiniums ?

If this happened early in the year, before they'd grown much, it'll be slugs/snails - they love them and will crop them down to the ground as they emerge, given the chance.

Slugs and snails or even caterpillars would have done the damage. Ants don't eat plants so are not a problem.

I cut up an old lemonade bottle to make a collar to protect the young delphinums. Once they get established the slugs seem to lose interest. Worked for me, anyhow :0)

Re your slug problem, I too have tried all slug traps, beer traps, copper etc. but then I discovered the Slug Bell and it's brilliant. It uses conventional or organic slug pellets and the beauty is they last up to 12 weeks guarding your veg or in my case growbags 24/7 365 days. You can water the garden as per normal as the pellets are not watered down by rain, mist or frost. They are brilliant as no pets can be harmed and when they have neutralised the area in question you can simply move it to another location.
You can get the Slug Bell from Rainbow Garden Products tel: 01932 221501. they are made of metal which last years and come in a flower design to blend in with the garden, they cost £7.99 each.
The website I have been told is due to launch at the beginning of October. Give them a go, it is the best investment I ever made " and I love my garden"

do they work on snails?

Any upturned pot with no holes in the bottom (which has become the top) will work there is no need to go and buy something just to catch slugs.