Yeh the old O ring definitely came out, I compared it with the new one to make sure they were the same. then I definitely put the new one in.
After checking this morning though, the leak seems that have slowed down. I drove home after work yesterday (about 30 mins) and only had to top up about 1/4 of a litre, then drove in this morning (over an hour i rush hour traffic) and no coolant warning light.
I'll check later to see how much coolant I lost, but it's strange that the leak has slowed, because before after driving off, the warning light would come on after about 10 mins.