The 520 has adjusters for the mechs coming out of the sides of the hoods. The front mech adjuster is the one on your left. Turn it backwards to move the mech outwards, roll it forwards to move the mech inwards. If the adjusters are at their limits, and the chain is still rubbing, you need to tweak the tension of the gear cable up a bit. Put the bike in its lowest tension setting ( chain on the smallest ring and smallest sprocket) then release the cable pinch bolt on the front mech. Pull the cable taut with a pair of pliers, and retigthen the bolt. You should get enough response from the adjuster on the hood, to move the mech enough, to stop the chain rub.
It's not really the shops fault. They can set it up so it's working just fine, and within a couple of hours riding, the cables / housings etc. bed in, they aren't in the position that was set up for originally, and then you have to adjust bits to account for the bedding in process. It's why they have a free check up and adjust service ( 6 weeks IIRC) after you buy the bike, you don't even have to keep the receipt, if you've registered for rewards points ( worth having from Decathlon). Nearly all reputable bike shops have something similar in place.
For the benefit of the community, this is not good advice. (I assume as a result of repeated bad experience setting up his gears or getting his LBS to do it for him).
Right, let's start at the beginning shall we. I know what I'm doing, I don't need the LBS to do it for me. My many years and hundreds of thousands of miles of riding experience come in handy for that sort of thing. It doesn't matter how well you set up the bike, things will shake about, and change positions slightly whatever you do. I didn't mention cable stretch, just position changes in components, due to 'shakedown'. My advice is good, and based on experience, and knowledge. In this particular instance, knowledge and experience of the actual model of bike ( gained from years of ownership of a Triban 520).
