I have a theory that most kids this age go through that, and I reckon it's all tied into teething. It's just my own theory, like, but I remember both mine being like that. Capol didn't make much difference in that it didn't work quite enough to let them go to sleep anyway. I reckon it worked just enough to stop them actually crying from the pain, but not enough to actually sleep. The only comfort seems to come from a parent staying with them. Understandable if they DO have a pressure in their jaws/head with the teething.
We had a couple of bad nights with both of mine where calpol didnt' even stop the crying. They were seemingly inconsolable and writhing round crying. Cuddles didn't help. Nelsons teething powders were the only thing which helped calm them down in those situations.
Like I say, I think teething has a lot to answer for, and I think it's easy to say "Well, he's had Calpol and still not settling so it can't be teething, therefore he's just playing me up." I just think that it's a bit more complicated than that. But then I am no medical expert whatsoever, it's just what I've experienced with my own children. Only problem is, when you have a run of teething nights like that, they DO get used to you being there, and then when they are obviously NOT teething anymore, they still expect you to be with them at night and that's when you have to be a bit firm with them.
You might just have to grit your teeth and prepare yourself for nights like this from time to time.