Thanks for the reply, Verdun. I went out tonight and the head that flowered a few days ago had all the ends of its petals chewed off, and there was a veevil sat right on it. So, caught red handed I squashed it.
I was also overrun with snails and slugs this year, and they were steadily chewing away at the dahlia when it first appeared. Some of them were massive, especially the slugs. I took to going out every couple of nights and rounding up as many as I could. This worled quite well, and I had less of a problem after a couple of weeks. I also tried a beer trap, and that seemed to have worked very well too. However I think I need to put a beer trap every few feet for it to work properly. One morning I went out after not checking it for a couple of days and there was a thick 'layer' of small slugs floating on the top above the beer. I put them all in the compost bin.
I've cut off the dead heads I've noticed. Do these go in the compost bin, or could they be used to grow new dahlias as easily as buying new tubers? I don't have a greenhouse.