I have a 18mth old (and 2 older kids) and you mentioning that you had an overseas trip in this post stood out. I have found with all my children that it only takes a break such as a holiday, especially with bub sleeping with/near you, to change their sleep pattern. We recentl went away over Xmas, and my DS played up after we got home, as he wanted to continue having me in with him when he slpet!
I have found what I call the "slow withdrawal method" to get past it. I dont believe in CIO, but I have found co-sleeping just doesn't work for us (Bub too restless, so no-one gets any sleep), so I graduall over a week or so withdraw how much time I spend in the nursery.
I start by following the usual bedtime routine (dinner bath, bed etc), put him in his cot. If he cries I will go in and give cuddle, place him back in his cot, telling him its "nigh-nighs" time. If he cries again, I go in, sit on a chair by the cot and tell him I will give him a pat, and sit there and pat him til he is sleepy - I try not to stay til he is asleep, but sometimes I have had to. EAch night I pat less, just leaving my hand on him, then just sit next to the cot and "shhhh" him. I think you get the jist!!
I have used this with 3 kids and it has definitely helped them learn to sleep on their own. I do agree it is an age thing as well, all 3 of mine have had sleep disturbance at around that age, due to wanting me with them (sep. anxiety related I think).
Anyway, I hope that helps, something to try anyway