Hi guys, some help needed if you please!

It is a 2005 1.8 non vvt

Basically my vectra always starts, but it has a struggle to do so either hot or cold, when left for more than 15 mins up to all night. Almost sounds like its flooded when you go to start it and you get a wiff of fuel as it eventually fires up.
Once started it runs brilliantly without any problems. If you shut the engine off and turn the key again it starts first turn.
I have had the car at a vauxhall garage this week and so far they have checked the basics such as fault logs, channel 21, software mod status etc..
Nothing has come up to say that anything is at fault so far and they are a little confused.
Clutching at straws i know, but i just thought i would ask if anyone might have an idea.