Cold start problem

[blueST]

My Wife has a 2009 W169 Mercedes A Class 1.5 petrol. I drove it for the forst time in ages yesterday, when I came to start it it turn over and catches immediately, but then immediately stops. Turns out the only way to get it started is to use a little bit of throttle. On asking my Wife, turns out she hasn't noticed anything as she always starts it like that! It definitely wasn't doing it when I last drove it a few months ago.

Once started it seems to run perfectly normally, and if it's not stone cold it will start fine with no throttle. Any ideas?

[Megaflow]

Sticky idle valve would be a good place to start, good clean with carb cleaner.