Thanks Hugh, I agree with you and I think it's teething plus the 4 month sleep regression. Killer combo! 
I'm not comfortable leaving him to cry, low-level whinging when he's falling asleep is fine, but when it turns into distressed crying I don't want to leave him, I find it too distressing myself. It's bad enough in the car when I can't do anything about it!
I don't mean to criticise anyone else though, it sounds like everyone has methods that work for them, I'm just not quite ready yet.
DS has rejected a dummy each time we've tried it which I think is a shame as that could have helped him to sleep, although i guess there is the issue of having to put it back in when it falls out!