Can't change into the largest sprocket on rear derailleur I bought my first road bike recently, and I've noticed I can't change into the largest cog/sprocket on the rear derailleur because when I pull the shifter the chain is pushed into the side of the largest sprocket and not over any of the teeth. 

If I (gently) use my thumb to push the arm (chain guide?) the chain will mount the largest sprocket but as soon as I let go with my thumb it jumps down to the second largest sprocket.

I'm guessing this isn't normal and somehow it hasn't been fitted quite right? Is there anything I can do to fix this or should I just take it back to the shop?
Just about any new bike you ever purchase will need a good tune up after the first 100 miles, not unlike a lot of cars or motorcycles. It's perfectly normal for most of the parts, especially parts under tension like brakes and cables, to settle and loosen a bit the first few times you ride the bike.

It's likely that the cable for the rear shifter has loosened a little bit and needs to be tightened, or the limit screw has settled. The shop you bought the bike from should have no problem performing the tune up for you, it's very common practice ( and if they try to charge you for it, hesitate to give them your business, the 100 mile tune-up should be complimentary ).