Thank you for your reply. When he's not overtired he goes to sleep without my help, for example this morning I put him down at 10am and he lay chatting to himself for 25 mins before falling asleep. He woke up 35 mins later and was chatting and gooing. He then starts to get frustrated if I don't go into get him, which is where I'm stuck.
If he's overtired when I try to put him down its a different story, I usually have to give him a dummy and keep popping it back in his mouth when he loses it. It drops out his mouth when he falls asleep and he doesn't notice though, he just stays asleep.
In extreme tiredness (usually by the afternoon) I don't stand a chance at getting him to sleep in his cot and instead I have to rock him to sleep in my arms, where he screams himself to sleep, the whole process takes around 30 mins and I don't try and put him down once he's asleep because it was such an effort to get him to sleep in the first place I'm scared I'll have to start all over again! Lol.