I had a similar situation to yours...
I was at a party, where they were serving some punch. As the party really got going, one girl decided it would be "funny" to put window cleaner in the punch. She was that out of it that she forgot she had done this a few minutes later, but her group of friends just watched and laughed. Meanwhile, a couple other people, including my boyfriend, had glasses of punch (with nobody in this group of friends saying anything), and made comments about how it tasted "funny" (but as they had already had more than a few to drink, they just kept drinking it). To make a long story short, the incident came to light, and I completely freaked out (I was sober).
I rang 999 in hysterics, and asked for 'poison control', not realising there isn't such a thing here in the UK. The 999 guy kept repeating that he could only put me through to the police, fire department or an ambulance (I didn't think I needed an ambulance, as everybody who had drunk the punch insisted they were 'fine', but didn't have the number for a hospital or GP's office in my phone in order to speak to a doctor). The 999 guy eventually put me through to the ambulance, where I explained the situation - they gave me the option of either sending an ambulance around, or calling NHS Direct (I was incredibly frustrated by this point, as I just needed someone to tell me that the window cleaner wasn't lethal!!!). I called the NHS Direct, who were incredibly helpful. They attempted toto look up the active ingredient in the cleaner, gave me a list of symptoms to look for, and told me which symptoms meant I needed to call an ambulance straight away. Basically, they put me at ease!
So moral of the story is - in ever in any situation of this sort, the NHS Direct is a good starting point (and it's a handy number to put in your phone!). I'm surprised that there isn't a poison control centre in the UK, like there is in the States. I was also surprised by the impression I got using 999 - that it is just a call centre that routes calls to the relevent departments. Maybe I used to watch too many Emergency, 911-type TV shows back home, but I thought the 999 operators would be trained to offer some basic medical advice and assistance, as well as forwarding the call!
Oh yeah... everybody who drank the punch was fine (albeit drunk)... thank god the cleaner didn't have any bleach in it!!!