If I were you Id be calling the vet pronto for advice. Most of what councils put down for poison these days isn't that harmful for dogs, in the sort of quantities you are talking about, however there is alot of the old (good) stuff still being used. If your dog is quiet and drinking loads that doesn't sound too good. We have lots of dogs and find that our gut feelings are usually right. You know your dog, don't hesitate if it is a potentially poison problem.
One of our lab ate a poisoned rat once and he was given a shot of vit k to bring it back up (he was a bit poorly afterwards) but with your situation I guess it would be hard to know where to start, given that it could be various poisons.
We have an emergency out of hours number at hours at ours and usually they discuss over the phone, to
assess before asking you to come in or they go out etc etc. Free advice given etc. I don't think this is
uncommon practice. Why don't you just call and see what they have to say? Best of luck, hope all goes well.