Hi bumbly - really feel for you as I had this with DS at about the same age. He'd slept really well from 6 weeks and I was feeling a bit smug then he started being scared of the dark and wanting me to stay with him till he fell asleep.
I tried loads of things from staying with him (I took a book and made it 'me' time!) till he fell asleep, shutting him in his room and refusing to let him out (awful, it didn't work and made me feel dreadful - don't think I managed a single night of this).
What has worked for us is for him to have a table lamp on - one of the cool touch ones - and the door open. I leave the landing light on but mainly so he can go to the loo and I can do the next bit without turning it on and off all the time. I stay for 10 minutes if he went to bed without playing up, or 5 minutes if he did, or its late for any reason. He has a story CD on to listen to but nothing exciting (we have Winnie the Pooh). After that I tell him I am going downstairs but will come back in 10 minutes to see how he is getting on. In that time he is allowed to get out of bed to go to the toilet or re-start the CD. I set a timer for 10 minutes so I can get on with stuff without forgetting about going back. If he comes downstairs I show him the timer which tells him how long I have been gone and depending on the excuse, send him back up to bed on his own for the remaining time, or stop the timer, take him back and start it again. When I go back I give him a quick kiss, tell him well done and that I will be back in another 10 minutes. Repeat until he's asleep.
That sounds long when I put it down like that but it isn't in practice. How long he takes to go to sleep varies a lot and sometimes I seem to do nothing but go up and downstairs but it does free me from his bedside and he does go off to sleep on his own, in his own bed.